Much more important, however, is the fact that the Egyptian texts bear witness to an “only begotten god” (meaning begotten of one parent only), whose symbol was the beetle because in ancient science this creature was thought to be “self-produced, being unconceived by a female.” Massey says, “The only begotten god is a well-known type [symbol], then, of divinity worshipped in Egypt. In each cult, the Messiah-son and manifestor was the only-begotten god. This, according to the Egyptian text, is the Christ, the Word, the manifestor in John’s Gospel.” In fact, in one early version of the Greek text of the New Testament’s Gospel of John, the phrase “the only begotten son of God” actually reads “the only begotten god”! Its very unorthodoxy makes it likely that it is the preferred, original reading.
The truth thus came forcefully home to me that this Egyptian Christ is indeed the express image of the Christ of John’s Gospel, who begins in the first chapter without father or mother and is the Word of the beginning, the opener and the architect, the light of the world, the self-originated and only-begotten God. I found that the very phraseology of John often echoed the Egyptian texts, which tell of he who was “the Beginning of the becoming, from the first, who made all things but was not made.” Some of the Fathers of the Church knew that the beetle was a symbol of Christ. Augustine, indeed, writes, “My own good beetle, not so much because he is only begotten (God), not because he, the author of himself, has taken on the form of mortals, but because he has rolled himself in our filth and chooses to be born from this filth itself” – like the dung beetle.
As Harpur is quoting Gerald Massey here, I assume he also found the quotes of Augustine within Massey’s writings. Massey does mention the Church Father Augustine and Ambrose as well. I looked around and found a site (linked below) where his work can be found along with helpful notes. The person who runs the site said they had some difficulty tracking down some of the references. Some apologists like to dismiss these quotes of Massey because he sometimes doesn’t offer citations (a problem with a lot of older scholarship).
In one rendition of John’s gospel, instead of the ‘only-begotten Son of God,’ a variant reading gives the ‘only-begotten God,’ which has been declared an impossible rendering. But the ‘only-begotten God’ was an especial type in Egyptian mythology, and the phrase re-identifies the divinity whose emblem is the beetle. Horapollo says, ‘To denote the only-begotten or a father, the Egyptians delineate a scarabaeus! [p.11]By this they symbolize an only-begotten, because the creature is self-produced, being unconceived by a female.’Now the youthful manifestor of the beetle-god was this Iu-em-hept, the Egyptian Jesus. The very phraseology of John is common to the inscriptions, which tell of him who was the Beginner of Becoming from the first, and who made all things, but who himself was not made. I quote verbatim. And not only was the beetle-god continued in the ‘only-begotten God’; the beetle-type was also brought on as a symbol of the Christ. Ambrose and Augustine, amongst the Christian Fathers, identified Jesus with, and as, the ‘good Scarabaeus,’ which further identifies the Jesus of John’s gospel with the Jesus of Egypt, who was the Ever-Coming One, and the bringer of peace, whom I have elsewhereshown to be the Jesus to whom the Book of Ecclesiasticus is inscribed, and ascribed in the Apocrypha.
In accordance with this continuation of the Kamite symbols, it was also maintained by some sectaries that Jesus was a potter, and not a carpenter; and the fact is that this only-begotten beetle-god, who is portrayed sitting at the potter’s wheel forming the egg, or shaping the vase-symbol of creation, was the potter personified, as well as the only-begotten god in Egypt.
 [Ambrose, Works, Paris, 1686, vol. 1, col. 1528. 'After the Christian era the influence of the scarab was still felt. St Ambrose, Archbishop of Milan, calls Jesus: "The good Scarabaeus, who rolled up before him the hitherto unshapen mud of our bodies."' See Myers, Scarabs, p. 63. See also BB 1:233, BB 2:317, NG 2:408. See AE 2:732 where both this quote and the above are cited on the same page.]
Following that citation, I found some quotes of the Church Fathers in Isaac Myer‘s book Scarabs on p. 63:
After the Christian era the influence of cult of the scarab was still felt. St. Ambrose, Archbishop of Milan, calls Jesus: “The good Scarabaeus, who rolled up before him the hitherto unshapen mud of our bodies.” St. Epiphanius has been quoted as saying of Christ: “He is the scarabaeus of God,” and indeed it appears likely that what may be called, Christian forms of scarab, yet exist. One has been described as representing the crucifixion of Jesus; if is white and engraving is in green on the back are two palm branches; many others have been found apparently engraved with the Latin cross.
Myers gives this citation: Works, Pris, 1686, Vol. I., col.1528, No. 113. Egyptian Mythology and Egyptian Christianity, etc., by Samuel Sharpe. London, 1863, p. 3. In Samuel Sharpe’s book, I could only find the quotes on p. 111 near the end of the chapter titled The Religion of Lower Egypt but there is no citation:
St. Ambrose, Archbishop of Milan, calls Jesus “the good Scarabaeus, who rolled up before him the hitherto unshapen mud of our bodies,” thus giving to him one of the names and characters of the god Horus, who is pictured as a scarabaeus with a ball of mud between his feet. The ball, which usually means the sun, would seem to have sometimes meant the sins of mankind; and the goddesses Isis and Nephthys are represented as rolling the same ball before them. St. Augustin also during the greater part of his life was a Manichaean, and held the Gnostic opinion of a god of goodness and a god of evil; and he was so far an admirer of the Egyptians, or at least of their practice of making mummies, as to say that they were the only Christians who really and fully believed in a future resurrection from the dead.
Also referring to Myer’s book is The Evolution of the Idea of God by Grant Allen and Franklin T. Richards (page 145):
In Mr. Loftie’s collection of sacred beetles is a scarabaeus containing a representation of the crucifixion, with two palm branches: and other scarabs have Christian crosses. If we remember how extremely sacred the scarab was held in the Egyptian religion, and also that it was regarded as the symbol of resurrection, we cannot possibly miss the importance of this implication. Indeed, the Alexandrian Father, Epiphanius, speaks of Christ as “the scarabaeus of God,” a phrase which may be still better understood if I add that in the treatise on hieroglyphs known under the name of Horapollo a scarabaeus is said to denote “an only-begotten.” Thus “the lamb of God” in the tongue of Israel becomes “the scarabaeus of God” in the mouth of an Egyptian speaker.
I also came across a reference in the Encyclopedia Britannica, 9th Edition (1875) and 10th Edition (1902). In the article Alchemy (Part 2), this is written:
In Egypt the doctrine of the Palingenesis was symbolized by the Scarabeaus, which suggested to St Augustine the following strange comparison: “Jesus Christus bonus ille scarabaeus meus, non ea tantum de causa unigenitus, quod, ipsement sui auctor mortalium speciem induxerit, sed quod in fac faece nostra sese volutarit et ex ipsa nasci homo voluerit.”
A favorite quotation of the Fathers was “He reigned from the wood” which they applied to Christ. The words “from the wood” are an addition found in some Mss. of the Seventy in Ps. xcvi. 10; and from the old Latin version the reading found its way into the pages of Tertullian.
In Hab. ii. 11, the Seventy render the word “beam” . . . but probably it merely meant a knot in the wood.  Some Latin versions rendered it “scarabaeus,” beetle, and this led to some singular comments. Thus St. Ambrose (De Obitu Theodosii) speaks of “Him who, like a beetle, called to his persecutors,” and says “He was the good beetle who called from the wood.” 
 Vulg., Lignum quod inter junctivas aedificiorum est (tie-beam).
 On Luke xxiii. We find elsewhere “bonus scarabaeus” applied to our Lord.
In The Expositor, this issue of the Septuagint (abbreviated LXX.) is also described on pages 25-26:
There are allusions and quotations in the ancient Fathers which, apart from the LXX., would be wholly unintelligible. When, for instance, St. Ambrose, in his orations De Orbita Theodosii, says of Helena, “She worshipped Him who hung on the wood; . . . . Him who, like a beetle, called to his persecutors,” &c. ; and in his comment on Luke xxiii., “He was the good beetle who called from the wood”—how utterly should we be at a loss to explain the allusion, if the LXX. did not furnish us with the requisite clue. In Hab. ii. 11, instead of “the beam out of the timber shall answer it,” we read in the LXX., . . . . which usually means “beetle,” is explained by St. Cyril to be a technical term for ” a cross-beam.” Hence “bonus scarabaeus,” “the good beetle,”—astonishing as such a title may appear to us,—was not unknown to Christian antiquity as a designation of our Lord. Again, when we find Tertullian challenging Marcion to tell him what he thought of David’s prophecy, “He reigned from the wood,” how much we should be perplexed to conjecture where any such prophecy occurred in the Old Testament, . . . . This reading found its way into the old Latin version, the Vetus Itala, and is referred to not only by Tertullian, but also by Justin Martyr.
“Bonus ille scarabaeus meus,” says St. Augustine, “non ea tantum de causa quod unigenitus, quod ipsemet sui auctor mortalium speciem induerit, sed quod in bac nostra faece sese volutaverit, et ex hae ipsa nasci voluerit.”
I noticed in the book Notes and Queries published by Oxford University Press in 1884 that someone had questioned about this (page 247):
In Moore’s Epicurean (third edition, 1827, p.313), there is a quotation from St. Augustine: “Bonus ille scarabaeus meus,” &c. I have searched the works of Augustine in vain to find this passage. Moore does not give any more exact reference. . . . In Migne’s edition (vol. v. col. 2039) there is a kind of abstract of a sermon, which may or may not be by St. Augustine, in which there is this sentence: “Christus in cruce vermis et scarabaeus.”
Robert Shaw, writing around the same time as Gerald Massey, came to similar conclusions in his book Sketch of the Religions of the World on pages 232-33:
In one version of Jno. 1, 18, instead of the “only begotten son” of God, the reading is the “only begotten God;” and it has been declared impossible for the sacred writer to have employed the phrase “only begotten God.” It is said to be contrary to the genius of the Gospel and opposed to the general teachings of the New Testament. But these things can only be determined by the doctrines and the gnosis that were pre-extant. Of course, the current Christology knows nothing of any such possible variant as the “only begotten God,” because of the ignorance of the Egyptian origines. But the “only begotten God” was an expecial type in the ancient allegory and the phrase recovers the divinity whose emblem is the beetle. This was Kephr-Ptah, who, like Atum, was reborn as his own son, Iu-em-hept, the Egyptian Jesus. “To denote an only begotten son or a father,” says Hor-Apollo, the Egyptians “delineate a Scarabeus. and they symbolize by this an only-begotten, because the scarabaeus is a creature self-produced, being unconceived by a female.” This was in a cult which tried hard to dethrone the female and exalt the male god as the only one. The “only begotten god” is a well-known gype of divinity in Egypt, worshipped as Khepr-Ptah and Khepr-Atum, and in each cult the Messiah, son and manifestor, was the only begotten god, Iu-em-hept, and Iu, the son whether of Ptah or Atum is Iusu or Jesu. This, according to the text, is the Christ, the Word, the Manifestor of John’s Gospel, who begins in the first chapter without father or mother, and is the Word of the beginning, the opener and architect, the light of the world, the self-originated and only begotten God. The phraseology of John is common in the Egyptian texts, which tell of him who was the Beginner of Becoming from the first, “who made all things but was not made.” There were Christian traditions which support this reading “only begotten God.” Some of the Fathers, Ambrose, for one, knew that the beetle was a symbol of Christ. Augustine also identifies the Christ with or as the good Scarabaeus, of which he speaks as follows: “He is my own good beetle, not because he is only-begotten, not because he himself, the author of himself, has taken on the form of mortals, but because he has rolled himself in our filth and chooses to be born from this filth itself.”
I noticed some authors mentioning Athanasius Kircherius. He apparently is the same as Athanasius Kircher who supposedly is considered the founder of Egyptology. Robert Taylor mentions him (along with others) on pages 11-12 in his book Devil’s Pulpit:
So the learned father Athanasius Kircherius assures us, that “by the May-bug was signified the only begotten Son of God, by whom all things were made, and witout whom was not anything made that was made.” The words of St. Augustin are: “Bonus ille scarabaeus meus, non ea tantum de causa, quod unigenitus, quod ipsemet sui auctor, mortalium speciem induerit, sed quod in hac faece nostra sese volutaverit, et ex ipsa, nasci homo voluerit. He [that is Jesus Christ] was my good cockchafer; not merely because, like a cockchafer, he was the only begotten, because he created himself, and put on a species of mortals, but because he created himself, and put on a species of mortals, but because he rolled himself, in human excre—” Casalius de. Veter. AEgyp. Ritibus, p. 35.) . . . . The learned Casalius, in quoting so solemn a declaration of so great a saint, that “Jesus Christ was a cockchafer, or May-bug,” proves that the saint must have been right, from those words of God himself, in the 22d Psalm, where he expressly says of himself—”as for me, I am a worm and not a man.”— . . . . where the Hebrew word, which has been translated, a worm, as the great Casalius thinks, should have been translated a cockchafer.
I couldn’t find anything about Casalius, but I found some more of Taylor’s writings in The Comet by H.D. Robinson. In connection with Kircherius’ statement about the may-bug/scarab, Taylor makes some interesting points on page 264 that give further context:
This Zodiacal worm, like all the rest of the signs of the Zodiac, was, in its turn, worshipped as the Supreme God, and it is none other than the most intelligent fathers of the Christian church, who assure us that it was Jesus Christ himself, who, in 22d Psalm, contemplating his descent into the lower regions, spoke in this character: ‘But as for me, I am a worm: and no many, a very scorn of men, and the outcast of people. Psalm xxii. 6.
Many of our learned translators render the word . . . . scarabaeus, or cockchafer, and one of the titles of Hercules was Scarabaeus, or Hercules, the cockchafer. But it is Christian, and not Pagan piety, to which we owe this sublime interpretation.