John Calvin 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 you continue in my word. Here Christ warns them, first, that it is not enough for anyone to have begun well if their progress to the end does not correspond to it; and for this reason, he exhorts to perseverance in the faith those who have tasted his doctrine. When he says that those who are firmly rooted in his word, so as to continue in him, will truly be his disciples, he means that many profess to be disciples who yet are not so in reality and have no right to be regarded as such.

He distinguishes his followers from hypocrites by this mark: those who falsely boasted of faith give way as soon as they have entered the course, or at least in the middle of it, but believers persevere constantly to the end. If, therefore, we wish that Christ should count us as his disciples, we must strive to persevere.