Adam Clarke on Acts 2:35

Until I make thy foes thy footstool - It was usual with conquerors to put their feet on the necks of vanquished leaders, as emblematical of the state of subjection to which they were reduced, and the total extinction of their power. By quoting these words, Peter shows the Jews, who continued enemies to Christ, that their discomfiture and ruin must necessarily take place, their own king and prophet having predicted this in connection with the other things which had already been so literally and circumstantially fulfilled. This conclusion had the desired effect, when pressed home with the strong application in the following verse.