John Calvin on Ephesians 1:13: “Our minds never become so firmly established in the truth of God as 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 frail and unsteady, if it rested on human wisdom; and therefore, as preaching is the instrument of faith, so the Holy Spirit makes preaching efficacious.”
George Smeaton: “… the impress of a seal implies a relation to the owner of the seal, and is a sure token of something belonging to him. From the three passages where the term ‘seal’ is expressly used, we gather that believers are God’s inviolable property, and known to be so by the Spirit dwelling in them. The sealing implies that the image engraven on the seal is impressed on the thing, or on the person sealed. In this case it is the image of God impressed on the heart by the enlightening, regenerating, and sanctifying power of the Holy Sprit. By that seal believers are declared to be the inviolable property of God (II Tim. 2:19); and they are sealed to the day of redemption as something which is known to be inviolably secure as God’s property (Eph. 4:30)” (The Doctrine of the Holy Spirit, pp. 44-45).
Reformed Doctrine Daily Devotions 