For this is the word of promise - This is the promise made to Abraham. The design of the apostle, in introducing this, is doubtless to show to whom the promise appertained; and by specifying this, he shows that it had not reference to Ishmael, but to Isaac.
At this time - Greek, According to this time; see Genesis 18:10, Genesis 18:14. Probably it means at the exact time promised; I will fulfil the prediction at the very time; compare 2 Kings 4:16.