John Chrysostom, on 1Cor 1:9

"God is faithful, by whom ye were called unto the fellowship of His Son." Wonderful! How great a thing saith he here! How vast in the magnitude of the gift which he declares! Into the fellowship of the Only-Begotten have ye been called, and do ye addict yourselves unto men? What can be worse than this wretchedness? And how have ye been called? By the Father. For since "through Him," and "in Him," were phrases which he was constantly employing in regard of the Son, lest men might suppose that he so mentioneth Him as being less, he ascribeth the same to the Father. For not by this one and that one, saith he, but "by the Father" have ye been called; by Him also have ye been "enriched." Again, "ye have been called;" ye did not yourselves approach. But what means, "into the fellowship of His Son?" Hear him declaring this very thing more clearly elsewhere. "If we suffer, we shall also reign with Him; if we die with Him, we shall also live with Him." Then, because it was a great thing which He had said, he adds an argument fraught with unanswerable conviction; for, saith he, "God is faithful," i.e. "true." Now if "true," what things He hath promised He will also perform. And He hath promised that He will make us partakers of His only-begotten Son; for to this end also did He call us. For "His gifts, and the calling of God," are without repentance.

These things, by a kind of divine art he inserts thus early, lest after the vehemence of the reproofs they might fall into despair. For assuredly God's part will ensue, if we be not quite impatient of His rein. As the Jews, being called, would not receive the blessings; but this was no longer of Him that called, but of their lack of sense. For He indeed was willing to give, but they, by refusing to receive, cast themselves away. For, had He called to a painful and toilsome undertaking, not even in that case were they pardonable in making excuse; however, they would have been able to say that so it was: but if the call be unto cleansing, and righteousness, and sanctification, and redemption, and grace, and a free gift, and the good things in store, which eye hath not seen, nor ear heard; and it be God that calls, and calls by Himself; what pardon can they deserve, who come not running to Him? Let no one therefore accuse God; for unbelief cometh not of Him that calleth, but of those who start away from Him.