Albert Barnes Commentary John 8:31

"Jesus therefore said to those Jews that had believed him, If ye abide in my word, [then] are ye truly my disciples;" — John 8:31 (ASV)

If ye continue in my word. If you continue to obey my commandments and to receive my doctrines.

Then are ye, etc. This is the true test of Christian character. John 14:21. He that hath my commandments and keepeth them, he it is that loveth me. See 1 John 2:4; 1 John 3:24; 2 John 1:6.

In this place Jesus cautions them against too much confidence from their present feelings. They were just converted—converted under a single sermon. They had had no time to test their faith. Jesus assures them that if their faith should abide the test, if it should produce obedience to his commandments and a holy life, it would be proof that their faith was genuine, for the tree is known by its fruit.

So we may say to all new converts: Do not repress your love or your joy, but do not be too confident. Your faith has not yet been tried, and if it does not produce a holy life, it is vain (James 2:17–26).