2 Corinthians 1:21

God, indeed, is always true and steadfast in his promises, and has always his Amen, as often as he speaks. But as for us, such is our vanity, that we do not utter our Amen in return, except when he gives a sure testimony in our hearts by his word. This he does by his Spirit. That is what Paul means here. He had previously taught, that this is a befitting harmony — when, on the one hand, the calling of God is without repentance, (Romans 11:29,) and we, in our turn, with an unwavering faith, accept of the blessing of adoption that is held out to us. That God remains steadfast to his promise is not surprising; but to keep pace with God in the steadfastness of our faith in return — that truly is not in man’s power. (294) He teaches us, also, that God cures our weakness or defect, (as they term it,) when, by correcting our belief, he confirms us by his Spirit. Thus it comes, that we glorify him by a firm steadfastness of faith. He associates himself, however, with the Corinthians, expressly for the purpose of conciliating their affections the better, with a view to the cultivation of unity. (295)

21.Who hath anointed us. He employs different terms to express one and the same thing. For along with confirmation, he employs the terms anointing and sealing, or, by this twofold metaphor, (296) he explains more distinctly what he had previously stated without a figure. For God, by pouring down upon us the heavenly grace of the Spirit, does, in this manner, seal upon our hearts the certainty of his own word. He then introduces a fourth idea — that the Spirit has been given to us as an earnest — a similitude which he frequently makes use of, and is also exceedingly appropriate. (297) For as the Spirit, in bearing witness of our adoption, is our security, and, by confirming the faith of the promises, is the seal ( σφραγὶς), so it is on good grounds that he is called an earnest, (298) because it is owing to him, that the covenant of God is ratified on both sides, which would, but for this, have hung in suspense. (299)

Here we must notice, in the first place, the relation (300) which Paul requires between the gospel of God and our faith; for as every thing that God says is more than merely certain, so he wishes that this should be established in our minds by a firm and sure assent. Secondly, we must observe that, as an assurance of this nature is a thing that is above the capacity of the human mind, it is the part of the Holy Spirit to confirm within us what God promises in his word. Hence it is that he has those titles of distinction — the Anointing, the Earnest, the Comforter, and the Seal. In the third place we must observe, that all that have not the Holy Spirit as a witness, so as to return their Amen to God, when calling them to an assured hope of salvation, do on false grounds assume the name of Christians.