A. A. Hodge on the sacraments as “seals” of the covenant. I found it helpful:
The sacraments were designed to ‘apply’—i.e., actually to convey—to believers the benefits of the new covenant. If they are ‘seals’ of the covenant, they must of course, as a legal form of investiture, actually convey the grace represented to those to whom it belongs. Thus a deed conveys an estate, or the key handed over in the presence of witnesses the possession of a house from the owner to the renter. Our Confession is explicit and emphatic on this subject. The old English word ‘exhibit,’ there used, does not mean to show forth; but, in the sense of the Latin exhibere, from which it is derived, to administer, to apply.
This Confession carefully guards in the third section of this chapter, showing that the sacraments have no inherent power or virtue at all, but that the right use of the sacrament is by divine appointment the occasion upon which the Holy Ghost conveys the grace to those to whom it belongs. So that this grace-conferring virtue depends upon two things: (1.) The sovereign will and power of the Holy Spirit. (2.) The lively faith of the recipient. The sacrament is a mere instrument; but IT IS AN INSTRUMENT OF DIVINE APPOINTMENT.
–A. A. Hodge, Westminster Confession: A Commentary, p. 331-332.