Our Lord wished to try the faith of those who believed, that it might not be only a superficial belief: Then said Jesus to those Jews which believed on Him, If ye continue in My word, then are ye My disciples indeed. His saying, if ye continue, made it manifest what was in their hearts. He knew that some believed, and would not continue. And He makes them a magnificent promise, viz. that they shall become His disciples indeed; which words are a tacit rebuke to some who had believed and afterwards withdrawn.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: Aug. [Chrys. Nic.] Hom. liv. 1