For the scripture saith; And Abraham circumcised of his household eighteen males and three hundred. What then was the knowledge given unto him? Understand ye that He saith the eighteen first, and then after an interval three hundred. In the eighteen 'I' stands for ten, 'H' for eight. Here thou hast JESUS (IHSOYS). And because the cross in the 'T' was to have grace, He saith also three hundred. So He revealeth Jesus in the two letters, and in the remaining one the cross. He who placed within us the innate gift of His covenant knoweth; no man hath ever learnt from me a more genuine word; but I know that ye are worthy [Barnabas 9:5 -9]
This information does not come from any known 'scripture' but nevertheless seems to come from the same lost source as what is recorded in Genesis Rabbah 84:4
וישב יעקב וגו', אברהם גייר גיורים, הה"ד (בראשית י''ב) ויקח אברם את שרי אשתו...ואת הנפש אשר עשו בחרן. 'אשר עשה' אין כתיב כאן אלא 'אשר עשו' - א"ר חוניא אברהם היה מגייר את האנשים ושרה מגיירת את הנשים, ומה ת"ל אשר עשו אלא מלמד שהיה אברהם אבינו מכניסן לתוך ביתו ומאכילן ומשקן ומקרבן ומכניסן תחת כנפי השכינה
יעקב גייר גיורים, דכתיב (בראשית ל''ה) ויאמר יעקב אל ביתו וגו' ויתנו אל יעקב וגו'. ביצחק לא שמענו והיכן שמענו? רבי יצחק ותאני לה משום רבי הושעיא רבה בשם ר' יהודה בר סימון: כאן כתיב, וישב יעקב בארץ מגורי אביו (בראשית ל''ז) מאי מגורי אביו מגיורי אביו
"Jacob settled [in the land of his father's sojournings, in Canaan." (Gen. 37:1) Abraham converted proselytes, as it is written, "Abraham took Sarai his wife . . . and the souls that they had made in Haran." (Gen. 12:5) . . . 'That he had made' is not written, rather 'that they had made.' R. Hunia said: Abraham converted the men and Sarah the women. That phrase also teaches us that our ancestor Abraham would bring them into his house, feed them, give them drink, and bring them nearer to the presence of God.
The point is that this clearly a baptism reference given that ritual water immersion preceded circumcision (or at least that seems to be the point of the author of Barnabas).
I hesitate to fully explain this passage to my non-Jewish readers but the content here not only references the 'circumcision' of Abraham's 318 proselytes but more interestingly the underlying reference in Clement and Barnabas to 'the cross' or tree from this passage. The Talmud explains that the Hebrew word for tamarisk is eshel, whose three letters (aleph, shin, and lamed) stand for food (ochel), drink (shtiyah), and lodging or escort (lina or levaya) which references these same ideas about Abraham's hospitality leading to converts to Judaism.
The sign of the cross and the name 'Jesus' being employed as the initiates stand in the water. The specific number 318 is drawn from som unknown 'scripture' but it has to be related to the name 'Lazarus' (Eliezer) given the underlying connection with Abraham. The point seems to be that Christian baptism with its invocations was a 'fulfillment' of Abraham's conversion of 318 souls in 'the Bible.'
