The Seal of Earnest.
I. The first metaphor in the text, the "seal" of the Spirit. A seal is impressed upon a recipient material made soft by warmth, in order to leave there a copy of itself. The Spirit of God comes into our spirits, and by gentle contact impresses upon material, which was intractable until it was melted by the genial warmth of faith and love, the likeness of itself; but yet so as that prominences correspond to the hollows, and what is in relief in the one is sunk in the other.
II. Note the "earnest" which consists in like manner "of the Spirit." The "earnest," of course, is a small portion of purchase-money, or wages, or contract-money, which is given at the completion of the bargain as an assurance that the whole amount will be paid in due time. "And," says the Apostle, "this seal is also an earnest." It not only makes certain God's ownership and guarantees the security of those on whom it is impressed, but it also points onwards to the future, and at once guarantees that and to a certain extent reveals the nature of it. You have but to take the faith, the love, the obedience, the communion, of the highest moments of the Christian life on earth, and take from them all their limitations, subtract all their imperfections, and stretch them out to absolute eternity, and you get heaven. The earnest is of a piece with the inheritance.
A. Maclaren, The Unchanging Christ,p. 104.