2 He heard the opposite of the promises from Him who had made them; and yet he was not disturbed, but did them as if they had been in harmony [therewith]. For they were in harmony; being opposed indeed according to human calculations, but in harmony [when viewed] by Faith. And how this was, the Apostle himself has taught us, by saying, “accounting that God was able to raise Him up, even from the dead.” By the same faith (he means) by which he believed that God gave what was not, and raised up the dead, by the same was he persuaded that He would also raise him up after he had been slain in sacrifice. For it was alike impossible (to human calculation, I mean) from a womb which was dead and grown old and already become useless for child-bearing to give a child, and to raise again one who had been slain. But his previous faith prepared the way for things to come.
And see; the good things came first, and the hard things afterwards, in his old age. But for you, on the contrary, (he says) the sad things are first, and the good things last. This for those who dare to say, 'He has promised us the good things after death; perhaps He has deceived us.' He shows that “God is able to raise up even from the dead,” and if God be able to raise from the dead, without all doubt He will pay all [that He has promised].
But if Abraham so many years before, believed “that God is able to raise from the dead,” much more ought we to believe it. You see (what I at first said) that death had not yet entered in and yet He drew them at once to the hope of the resurrection, and led them to such full assurance, that when bidden, they even slay their own sons, and readily offer up those from whom they expected to people the world.
And he shows another thing too, by saying, that “God tempted Abraham.” What then? Did not God know that the man was noble and approved? Why then did He tempt him? Not that He might Himself learn, but that He might show to others, and make his fortitude manifest to all. And here also he shows the cause of trials, that they may not suppose they suffer these things as being forsaken [of God]. For in their case indeed, it was necessary that they should he tried, because there were many who persecuted or plotted against them: but in Abraham's case, what need was there to devise trials for him which did not exist? Now this trial, it is evident, was by His command. The others indeed happened by His allowance, but this even by His command. If then temptations make men approved in such wise that, even where there is no occasion, God exercises His own athletes; much more ought we to bear all things nobly.
And here he said emphatically, “By faith, when he was tried, he offered up Isaac,” for there was no other cause for his bringing the offering but that.
Source: Homilies on the Epistle to the Hebrews (New Advent)