I have finally finished my paper on Eternal Justification. While I do not claim to know everything on this issue, it is my hope that this paper would cover at least the main points regarding the issue of contention, which is indeed of importance with regards to the defence of the Gospel.
The doctrine of Eternal Justification is the theory that the Elect are justified before God from eternity. For adherents to this doctrine, the decree to justify the elect is from eternity, and so therefore from eternity the elect of God are considered as being justified, and this fact they term Eternal Justification. This is distinguished from the Justification that occurs in time which is by faith. Eternal Justification adherents typically believe in a two-fold justification, one active and objective which is by God from eternity, and the other passive and subjective which happens by faith. As stated by Gill
Justification is an act of God's grace, flowing from his sovereign good will and pleasure; the elect of God are said to be "justified by his grace"; and as if that expression was not strong enough to set forth the freeness of it, the word "freely" is added elsewhere; "Being justified freely by his grace", #Tit 3:7 Ro 3:24. Justification is by many divines distinguished into active and passive. Active justification is the act of God; it is God that justifies. Passive justification is the act of God, terminating on the conscience of a believer, commonly called a transient act, passing upon an external object. It is not of this I shall now treat, but of the former; which is an act internal and eternal, taken up in the divine mind from eternity, and is an immanent, abiding one in it; it is, as Dr. Ames expresses it, "a sentence conceived in the divine mind, by the decree of justifying."
There are therefore two kinds of justification being discussed: a "justification from eternity" which is an "immanent" (internal and eternal) act of God and a "justification by faith" which is a "transient" (acting externally in time) act. As Gills continues,
Now, as before observed, as God's will to elect, is the election of his people, so his will to justify them, is the justification of them; as it is an immanent act in God, it is an act of his grace towards them, is wholly without them, entirely resides in the divine mind, and lies in his estimating, accounting, and constituting them righteous, through the righteousness of his Son; and, as such, did not first commence in time, but from eternity.
In other words, because God is eternal and so timeless, his decree of election is the same as his electing of his people, as his decree of justification is the justification of them. God being immutable cannot change and thus his immanent decree originating from Himself cannot change either. According to this reasoning therefore, since God decreed to justify His people from eternity; it is an act of God and thus active; this decree must be immanent and therefore eternal. God being timeless therefore means that His decree must equate to the actions being "done" in God's perspective.