John Calvin Commentary Acts 1:17

"For he was numbered among us, and received his portion in this ministry." — Acts 1:17 (ASV)

Adopted. It is, word for word, reckoned. And he says that he was one of the number, so that he might signify to them that it was necessary for the empty place to be filled, in order that the number might remain whole. What follows serves this purpose: that he had obtained a part in the ministry.

For from this it follows that the body would be, as it were, lame, if that part were missing. Surely it was something that would greatly amaze them, that he whom Christ had exalted to such high dignity should fall headlong into such destruction.

This circumstance increases the cruelty of the act and teaches the others to take heed to themselves. Nor is it to be doubted that the disciples remembered Judas with great grief and sorrow. But Peter here expresses by name the excellence of his office, so that he might make them more attentive and careful to provide a remedy.