3 After this he pursues the same thought. No one (he says) could allege, that he had another son, and expected the promise to be fulfilled from him, and therefore confidently offered up this one. “And” (his words are) “he offered up his only-begotten, who had received the promises.” Why do you say “only-begotten”? What then? Of whom was Ishmael sprung? I mean “only-begotten” (he would say) so far as relates to the word of the promise. Therefore after saying, “Only-begotten,” showing that he says it for this reason, he added, “of whom it was said, In Isaac shall your seed be called,” that is, “from” him. Do you see how he admires what was done by the Patriarch? “In Isaac shall your seed be called,” and that son he brought to be sacrificed.
Afterwards, that no one may suppose he does this in despair, and in consequence of this command had cast away that Faith, but may understand that this also was truly of faith, he says that he retained that faith also, although it seem to be at variance with this. But it was not at variance. For he did not measure the power of God by human reasonings, but committed all to faith. And hence he was not afraid to say, that God was “able to raise him up, even from the dead.”
“From whence also he received him in a figure,” that is in idea, by the ram, he means. How? The ram having been slain, he was saved: so that by means of the ram he received him again, having slain it in his stead. But these things were types: for here it is the Son of God who is slain.
And observe, I beseech you, how great is His lovingkindness. For inasmuch as a great favor was to be given to men, He, wishing to do this, not by favor, but as a debtor, arranges that a man should first give up his own son on account of God's command, in order that He Himself might seem to be doing nothing great in giving up His own Son, since a man had done this before Him; that He might be supposed to do it not of grace, but of debt. For we wish to do this kindness also to those whom we love, others, to appear first to have received some little thing from them, and so give them all: and we boast more of the receiving than of the giving; and we do not say, We gave him this, but, We received this from him.
“From whence also” (are his words) “he received him in a figure,” i.e. as in a riddle (for the ram was as it were a figure of Isaac) or, as in a type. For since the sacrifice had been completed, and Isaac slain in purpose, therefore He gave him to the Patriarch.
Source: Homilies on the Epistle to the Hebrews (New Advent)