John Calvin Commentary Acts 20:25

"And now, behold, I know that ye all, among whom I went about preaching the kingdom, shall see my face no more." — Acts 20:25 (ASV)

And, behold, now I know. He now states plainly what he had previously hinted at subtly. And we said that he removed their hope of his return so that he might more deeply impress his exhortations upon their minds. For we know the great impact that the words and speeches of people have when uttered at their departure or death. Also, by this forewarning, he wanted them to be careful not to depend on his presence, lest their faith weaken through weariness.

The doctrine of the gospel is again called the kingdom of God. This doctrine begins the kingdom of God in this world by renewing people in the image of God, until it is finally perfected in the last resurrection.