Abraham, the father of us all
Romans 4:16b (KJV) ...Abraham; who is the father of us all,
By comparing Romans 4:9 with Ephesians 2:11 we know that "us all" means the circumcision and the uncircumcision, and that this in turn means Israel and Gentiles.
In our verse, Paul is concluding his argument defending the notion of justification before God by faith alone and without works, this being the foundational doctrine for this age. He had previously written:
Romans 3:21a, 22a, 28 (KJV) But now the righteousness of God without the law is manifested... Even the righteousness of God which is by faith of Jesus Christ...Therefore we conclude that a man is justified by faith without the deeds of the law.
Anticipating a Jewish interlocutor who would argue that Abraham demonstrates that justification is by works...
Romans 4:1 (KJV) What shall we say then that Abraham our father, as pertaining to the flesh, hath found?
...and to head any such argument off, Paul observes that righteousness was imputed to Abraham before circumcision (ie any works) was given, writing:
Romans 4:10 (KJV) How was it then reckoned? when he was in circumcision, or in uncircumcision? Not in circumcision, but in uncircumcision.
[Abraham counted righteous by faith:
Genesis 15:6 (KJV) And he believed in the LORD; and he counted it to him for righteousness.
Circumcision given:
Genesis 17:10 (KJV) This is my covenant, which ye shall keep, between me and you and thy seed after thee; Every man child among you shall be circumcised.]
This, then, is the sense in which Abraham is the father of us all - that he is the pattern for both Jews and Gentiles of justification by faith without works.
And Paul's wider point to the potential Jewish opponent is that Abraham demonstrates that the absence of a requirement for works (as is the case in this dispensation) does not in and of itself exclude the possibility of being justified before God.
Note: Although works began to be added after his justification for Abraham's physical seed with the requirement for circumcision (the necessity from then on being to demonstrate their faith by works), it was and will not be the works themselves which save Israel.