Teaology 41

8. Of Christ the Mediator

VI. Although the work of redemption was not actually wrought by Christ till after His incarnation, yet the virtue, efficacy, and benefits thereof were communicated unto the elect, in all ages successively from the beginning of the world, in and by those promises, types, and sacrifices, wherein He was revealed, and signified to be the seed of the woman which should bruise the serpent’s head; and the Lamb slain from the beginning of the world; being yesterday and today the same, and forever.

— The Westminster Confession of Faith

See also chapter 7, section 5 and 6.