2 Corinthians 1:21

But if repentance is a thing human, its baptism must necessarily be of the same nature: else, if it had been celestial, it would have given both the Holy Spirit and remission of sins. But none either pardons sins or freely grants the Spirit save God only. Even the Lord Himself said that the Spirit would not descend on any other condition, but that He should first ascend to the Father.

For this reason righteousness and peace are now far departed from you, inasmuch as every one abandons the fear of God, and has become blind in His faith, neither walks in the ordinances of His appointment, nor acts a part becoming a Christian, but walks after his own wicked lusts, resuming the practice of an unrighteous and ungodly envy, by which death itself entered into the world. [Wisdom 2:24]

After these words, after the renunciation of the devil and the covenant with Christ, inasmuch as you have henceforth become his very own and have nothing in common with that evil one, he straightway bids you to be marked and places on your forehead the sign of the cross. That savage beast is shameless, and when he hears those words, he grows more wild—as we might expect—and desires to assault you on sight. Hence God anoints your countenance and stamps thereon the sign of the cross. In this way God holds in check all the frenzy of the Evil One, for the devil will not dare to look on such a sight. Just as if he had beheld the rays of the sun and had leaped away, so will his eyes be blinded by the sight of your face and he will depart. For through the chrism the cross is stamped on you.… And that you may again know that it is not a man but God himself who anoints you by the hand of the priest, listen to St. Paul when he says: "It is God who makes both us and you stand firm in Christ, who has anointed us."

"Now He which stablisheth us with you in Christ, and anointed us, is God; Who also sealed us, and gave us the earnest of the Spirit in our hearts." Again, from the past He stablisheth the future. For if it is He that establisheth us in Christ; (i.e., who suffereth us not to be shaken from the faith which is in Christ;) and He that anointed us and gave the Spirit in our hearts, how shall He not give us the future things? For if He gave the principles and the foundations, and the root and the fount, (to wit, the true knowledge of Him, the partaking of the Spirit,) how shall He not give the things that come of these: for if for the sake of these those are given, much more will he supply those. And if to such as were enemies he gave these, much more when now made friends will He "freely give" to them those. Wherefore He said not simply "the Spirit," but named "earnest," that from this thou mightest have a good hope of the whole as well. For did He not purpose to give the whole, He would never have chosen to give "the earnest" and to waste it without object or result. And observe Paul's candor. For why need I say, saith he, that the truth of the promises standeth not in us? The fact of your standing unwavering and fixed is not in us, but this too is of God; "for" saith he, "He who stablisheth us is God." It is not we who strengthen you: for even we also need Him that stablisheth. So then let none imagine that the Preaching is hazardous in us. He hath undertaken the whole, He cared for the whole. And what is, "anointed," and "sealed?" Gave the Spirit by Whom He did both these things, making at once prophets and priests and kings, for in old times these three sorts were anointed. But we have now not one of these dignities, but all three preeminently. […]

Paul is saying that Christ confirms the Gentiles in the faith promised to the Jews, because he has made us both one.