Having maintained that the gospel is certain, the apostle Paul now comes to the proof. And what higher surety can be found than the Holy Spirit? Having denominated the gospel the word of truth, God will not prove it by the authority of men; for he has given us the testimony of the Spirit of God himself, the one who seals the truth of it in your hearts. Here we have a comparison taken from Seals, which had the effect of removing doubt on transactions among men. Seals gave validity both to charters and to testaments, they were the principal means by which the writer of a letter could be known; and, in short, a seal distinguishes what is true and certain, from what is false and spurious.
The apostle Paul uses the word “earnest” which means pledge, guarantee, a down payment. The Holy Spirit is given to the believer to give the believer perfect assurance of his salvation. We know that we are redeemed—that we are God’s cherished possession—by the Holy Spirit who lives within us. However, our minds never become so firmly established in the truth of God in order that we are able to resist all the temptations of Satan, until we have been confirmed in it by the Holy Spirit.
The true conviction which believers have of the word of God, of their own salvation, and of religion in general, does not spring from the judgment of the flesh, or from human and philosophical arguments, but from the sealing of the Spirit, who imparts to their consciences such certainty as to remove all doubt. The foundation of faith would be unsteady and easily fractured, if it rested on human wisdom; and therefore, as preaching is the instrument of faith, so the Holy Spirit makes preaching efficacious.
As the great puritan commentator, Matthew Henry states
“He (God) dispenses all his blessings, according to his good pleasure. His divine teaching led whom he pleased to see the glory of those truths, which others were left to blaspheme. — What a gracious promise that is, which secures the gift of the Holy Spirit to those who ask him! The sanctifying and comforting influences of the Holy Spirit seal believers as the children of God, and heirs of heaven. These are the first fruits of holy happiness. For this we were made, and for this we were redeemed; this is the great design of God in all that he has done for us; let all be ascribed to the praise of his glory”.
Again note: Why does God give us such a glorious guarantee as His own wonderful presence? That His glory might be praised eternally.