John 8:31

We have in these verses, I. A comfortable doctrine laid down concerning the spiritual liberty of Christ's disciples, intended for the encouragement of those Jews that believed. Christ, knowing that his doctrine began to work upon some of his hearers, and perceiving that virtue had gone out of him, turned his discourse from the proud Pharisees, and addressed himself to those weak believers. When he had denounced wrath against those that were hardened in unbelief, then he spoke comfort to those few feeble Jews that believed in him. See here, 1. How graciously the Lord Jesus looks to those that tremble at his word, and are ready to receive it; he has something to say to those who have hearing ears, and will not pass by those who set themselves in his way, without speaking to them. 2. How carefully he cherishes the beginnings of grace, and meets those that are coming towards him. These Jews that believed were yet but weak; but Christ did not therefore cast them off, for he gathers the lambs in his arms. When faith is in its infancy, he has knees to prevent it, breasts for it to suck, that it may not die from the womb. In what he said to them, we have two things, which he saith to all that should at any time believe: - (1.) The character of a true disciple of Christ: If you continue in my word, then are you my disciples indeed.

— Matthew Henry's Concise Commentary (public domain)

Then said Jesus to those Jews who believed, If ye continue in my word, then are ye my disciples indeed, &c.--The impression produced by the last words of our Lord may have become visible by some decisive movement, and here He takes advantage of it to press on them "continuance" in the faith, since then only were they His real disciples (compare Joh 15:3-8), and then should they experimentally "know the truth," and "by the truth be made (spiritually) free."

— Jamieson-Fausset-Brown Commentary (public domain)