In the old testament anyone who wanted to be a Jew the men had to be circumcised. That was the testimony that they believed the old testament. Paul said anyone who is circumcised(join in the old testament beliefs) any of them they are a debtor to believe in all of them. And they have fallen from grace.

[Gal 5:1-4
1 Stand fast therefore in the liberty by which Christ has made us free, and do not be entangled again with a yoke of bondage.
2 Indeed I, Paul, say to you that if you become circumcised, Christ will profit you nothing.
3 And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law.
4 You have become estranged from Christ, you who [attempt to] be justified by law; you have fallen from grace.
[Jhn 10:1, 8
1 "Most assuredly, I say to you, he who does not enter the sheepfold by the door, but climbs up some other way, the same is a thief and a robber. ...
8 "All who [ever] came before Me are thieves and robbers, but the sheep did not hear them