At this point, I understand that Abraham, the 'father of the Jews' was used by early Christian writers to craft the nascent Christian community. Paul referenced God acknowledging Abraham's faithfulness prior to his circumcision both in 15:6 when "Abram believed the LORD, and [God] credited it to him as righteousness" and earlier in Genesis 12:2-3 when God makes his first promise, and Paul, because these acknowledgements were made before Abraham is commanded to circumcise himself, claims that the promises of the covenant are open to people on the basis of faith. Barnabas uses typological arguments, drawing connections between stories in the old testament (some of which are related to Abraham) and Christ. These arguments supposedly show that the old testament, the 'type', is fulfilled and superseded by the 'anti-type' in Christ's life.
The main issue I have is that I can't understand why these writers would want to exclude the Jews (of whom many of them were a part) from the covenantal promises -- in some cases excluding them from the possibility of inclusion entirely.