Not as though the word of God hath taken none effect. For
they are not all Israel, which are of Israel: Neither,
because they are the seed of Abraham, are they all children: but, In
Isaac shall thy seed be called. That is, They which are
the children of the flesh, these are not the children of God: but the
children of the promise are counted for the seed. For this
is the word of promise, At this time will I come, and Sara shall have a
son. And not only this; but when Rebecca also had
conceived by one, even by our father Isaac; (For the
children being not yet born, neither having done any good or evil, that
the purpose of God according to election might stand, not of works, but
of him that calleth;) It was said unto her, The elder
shall serve the younger. As it is written, Jacob have I
loved, but Esau have I hated. What shall we say then? Is
there unrighteousness with God? God forbid. For he saith
to Moses, I will have mercy on whom I will have mercy, and I will have
compassion on whom I will have compassion. So then it is
not of him that willeth, nor of him that runneth, but of God that
sheweth mercy. For the scripture saith unto Pharaoh, Even
for this same purpose have I raised thee up, that I might shew my power
in thee, and that my name might be declared throughout all the earth.
Therefore hath he mercy on whom he will have mercy, and whom he will he
hardeneth. Thou wilt say then unto me, Why doth he yet
find fault? For who hath resisted his will?
I'm still finding myself enthralled with Chrysostom's homilies on Romans, so again will quote from this in with regard to today's reading.
"'For I could wish that I myself were accursed.' What does the 'I myself' mean? It means that I that have been a teacher (I Cor. 9:27) of all, that have gathered together countless good deeds, that am waiting for countless crowns, that desired Him so much, as to value His love above all things, who all my days am burning for Him, and hold all things (Phil 3:8) of second importance to the love of Him. For even being loved by Christ was not the only thing he cared for, but loving him exceedingly also. And this last he cared most for. So it was that he looked to this only, and took all things light-heartedly. For he kept one aim in view in all circumstances, the fulfilling of this excellent love." (Homily 16 on Romans)
I also want to link to an article written by Archimandrite (Metropolitan) Philaret "Will the Heterodox Be Saved" which quotes v15-16.
From Chrysostom's Homily 16:
"…there was one sin on which all the Jews joined, that of the molten calf, and still some were punished, and some were not punished. And this is why He says 'I will have mercy on whom I will have mercy, and I will have compassion on whom I will have compassion…" (Ex. 33:19)
(referring to v. 20 "Nay but, O man, who are you that repliest against God?" which I know isn't a part of today's reading, but I thought it was a good closing to the thought of this passage) "This he does to take down the objector's unseasonable inquisitiveness, and excessive curiosity, and to put a check upon it, and teach him to know what God is, and what man, and how incomprehensible His foreknowledge is, and how far above our reason, and how obedience to Him in all points is binding…For our business is to obey what God does, not to be curious even if we do not know the reason of them…Why in comparison with Him you cannot have a being even! Nor this or that sort of being, but absolutely none!…See how he scares them, how he terrifies them, how he makes them tremble rather than be questioning and curious. This is what an excellent teacher does; he does not follow his disciples' fancy everywhere, but leads them to his own mind, and pulls up the thorns, and then puts the seeds in, and does not answer at once in all cases to the questions put to him."