Theophylact of Ohrid

But Christ declares to them that even now they are still imperfect, that they have not understood anything great about Him, but are still thinking lowly and about earthly things. He says: "Do you now believe?" and by this He as it were reproaches and rebukes them for the slowness of their faith.

Commentary on John ↗

Augustine of Hippo

And still further admonishing them of their age as still small and infirm in regard to the inner man, "Jesus answered them: Do ye now believe? Behold the hour cometh, yea, is now come, that ye shall be scattered every man to his own, and shall leave me alone. And yet I am not alone, because the Father is with me." He had said shortly before, "I leave the world, and go to the Father;" now He says, "The Father is with me." Who goes to him who is with him? …

Tractates on John 103 ↗

Bede

Which can be understood in two ways, either as reproaching, or affirming. If the former, the meaning is, Ye have awaked somewhat late to belief, for behold the hour cometh, yea is now come, that ye shall be scattered every man to his home. If the latter, it is, That which ye believe is true, but behold the hour cometh, &c.

Catena Aurea by Aquinas ↗

Thomas Aquinas

2169 Now we see the condition of the disciples, which is one of weakness: first, Christ reproaches their slowness to believe; secondly, he foretells their approaching troubles; thirdly, he shows that he cannot be injured by them. 2170 As to the first he says, Do you now believe? If this is understood as a question, it is a rebuke for being so slow to believe. It is like saying: Have you waited till now to believe? If we understand this remissibly, Christ is reproving the instability of their faith. …

Commentary on John ↗