“Faith is the evidence and manifestation of justification, and therefore justification must be before it; "Faith is the evidence of things not seen", (Hebrews 11:1) but it is not the evidence of that which as yet is not; what it is an evidence of, must be, and it must exist before it. The "righteousness of God", of the God-man and mediator Jesus Christ, "is revealed from faith to faith", in the everlasting gospel, (Romans 1:17) and therefore must be before it is revealed, and before faith, to which it is revealed: faith is that grace whereby a soul, having seen its guilt, and its want of righteousness, beholds, in the light of the divine Spirit, a complete righteousness in Christ, renounces its own, lays hold off that, puts it on as a garment, rejoices in it, and glories of it; the Spirit of God witnessing to his spirit, that he is a justified person; and so he is evidently and declaratively "justified in the name of the Lord Jesus, and by the Spirit of our God" (1 Corinthians 6:11).
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Justification is not only before faith, but it is from eternity, being an immanent act in the divine mind, and so an internal and eternal one; as may be concluded,
2b1. From eternal election: the objects of justification are God's elect; "Who shall lay anything to the charge of God's elect? it is God that justifies"; that is, the elect. Now if God's elect, as such, can have nothing laid to their charge; but are by God acquitted, discharged, and justified; and if they bore this character of elect from eternity, or were chosen in Christ before the world began; then they must be acquitted, discharged and justified so early, so as nothing could be laid to their charge: besides, by electing grace men were put into Christ, and were considered as in him before the foundation of the world; and if they were considered as in him, they must be considered as righteous or unrighteous; not surely as unrighteous, unjustified, and in a state of condemnation; for "there is no condemnation to them which are in Christ", (Romans 8:1) and therefore must be considered as righteous, and so justified: "Justified then we were, says Dr. Goodwin when first elected, though not in our own persons, yet in our Head, as he had our persons then given him, and we came to have a being and an interest in him."
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2b3. Justification is one of those spiritual blessings with which the elect are blessed in Christ according to election-grace, before the foundation of the world (Ephesians 1:3 4). That justification is a spiritual blessing none will deny; and if the elect were blessed with all spiritual blessings, then with this; and if thus blessed according to election, or when elected, then before the foundation of the world: and this grace of justification must be no small part of that "grace which was given in Christ Jesus before the foundation of the world was" (2 Timothy 1:9).
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2b6. It was the will of God from everlasting, not to punish sin in the persons of his elect, but to punish it in the person of Christ; and that it was his will not to punish it in his people, but in his Son, is manifest from his setting him forth in his purposes and decrees, to be the atoning sacrifice for sin; and from his sending him forth in the likeness of sinful flesh, to condemn sin in the flesh; and from his being made sin and a curse, that his people might be made the righteousness of God in him. Now, as has been often observed, no new will can arise in God; God wills nothing in time, but what he willed from eternity; and if it was the eternal will of God not to punish sin in his people, but in his Son, then they were eternally discharged, acquitted from sin, and secured from everlasting wrath and destruction; and if they were eternally discharged from sin, and freed from punishment, they were eternally justified: Dr. Twisse makes the very quiddity and essence of justification and remission of sin, which he takes to be the same, to lie in the will of God not to punish; and asserts, that this will not to punish, as it is an immanent act, was from eternity.”
- excerpts from John Gill's A Body of Doctrinal Divinity