Such is the perfection of the atonement, that it corresponds to the justice of God revealed in the Word, to the demands of the law, and to the miseries and necessities of those for whom it was made. Had it been in its own nature deficient, and derived its sufficiency only from God’s acceptance of it through mere grace, then the victims under the law might have possessed equal efficacy in making atonement for sin, contrary to Heb. x. 4. Its perfection is derived from its own intrinsic fulness of merit. It is perfect: (1.) In respect to parts; because it satisfied all the demands which the law makes upon us, both in relation to the obedience of life and the suffering of death. By enduring the punishments due to us, it has freed us from death and condemnation. And by its meritorious efficacy, it has reconciled God the Father to us and has acquired for us a title to eternal life. (2.) It is perfect in degree; for Christ has not only done and suffered all that which the law claims of us, but all this in a full and perfect degree; so that nothing more, in this respect, can possibly be desired. The perfection in degree is derived from the infinite dignity of the person who suffered and the severity of the punishment exacted. (3.) Hence follows the perfection in its effects. In respect of God, it has effected an entire reconciliation with him; in relation to sin, it has wrought full expiation and pardon; and in relation to believers, its effects are perfection in holiness and complete redemption, both as to deliverance from death, and as to a title to life and its possession.
(Turretin, On the Atonement of Christ, 1859 ed. p. 68)
And Pictet writes:
And not without reason is this office assigned to faith, before all other graces, because it alone, out of all the others, can subsist or stand with divine grace, seeing that it is employed, as it were, in the mere receiving and aprehending of an object which is placed without it, and because, as Toletus a Papist observes, by faith it is more clearly shewn how man is justified, not by his own merit, but by the merit of Christ, and by it alone is “boasting excluded.”
(Pictet, Christian Theology, p. 370)