Philippians 3:3

For we are of the circumcision, who worship by the Spirit of God, and glory in Christ Jesus, and have no confidence in the flesh.

Both the Judaizers and the unbelieving Jews supported their opposition to Christianity by appealing to their circumcision as proof of their standing within the covenant of Abraham. They called themselves "the circumcision .... as proof of their right descent from Abraham and the only objects of God's favor.[13] Here Paul challenged them.

We are the circumcision ... Christians, not those fleshly descendants of Abraham, are the true Israel of God. This is a theme which Paul developed extensively in Romans; and for a further discussion of this subject see my Commentary on Romans, Romans 2:25ff and Romans 9:7ff.

MacKnight's paraphrase of this is:

The Judaizers, being destitute of the qualities signified by circumcision, have no title to the name and should be shunned. But we are the true circumcision, who worship God in Spirit and in truth, and boast in Christ Jesus as our Saviour, and have no reliance upon our descent from Abraham[14]

[13] James Macknight, Apostolical Epistles with Commentary, Volume III (Grand Rapids, Michigan: Baker Book House, 1969), p. 440.

[14] Ibid., p. 445.