John Calvin on Acts 1:17

17. Adopted. It is word for word reckoned. And he saith that he was one of the number, that he might signify unto them that it was needful that the empty place should be filled, to the end that the number might continue whole. And to this propose serveth that which followeth, that he had obtained a part in the ministry. For thereupon it doth follow that the body should be, as it were, lame, if that part should be wanting. Surely it was a thing which might make them greatly amazed, that he whom Christ had extolled unto so high dignity should fall headlong into such destruction. Which circumstance doth increase the cruelty of the fact, “Atrocitatem sceleris,” the atrocity of the crime. and teacheth the rest to take heed unto themselves. “Ut sibi caveant et metuani,” to be cautious and fear. Neither is it to be doubted but that the disciples did remember Judas with great grief and sorrow. But Peter doth here express by name the excellency of his function, that he might make them more attentive and more careful to provide a remedy.