"Applied retroactively"? Meaning the saving benefits of the new covenant were not able to be applied to Old Testament saints until the Lord actually shed His blood, rose from the dead, and ascended into heaven? That – from the past – He had to look into the future and send the blessings back into the past? That's like weird!
Let's not bring "time-travel" into the picture.
Agreed - let's not ever even think of God as existing in time. The One Who has no beginning or end created our reality of time ("In the beginning...").
Does this confessional statement (from WCF 3.5): "God, before the foundation of the world was laid, according to his eternal and immutable purpose, and the secret counsel and good pleasure of his will, hath chosen in Christ, unto everlasting glory, out of his mere free grace and love,
without any foresight of faith or good works, or perseverance in either of them, or any other thing in the creature, as conditions, or causes moving him thereunto; and all to the praise of his glorious grace" include foresight of Christ's "good works"? If so, doesn't this rule out His work being applied retroactively?
Also, "Man, by his fall, having made himself incapable of life by that covenant, the Lord was pleased to make a second, commonly called the covenant of grace; wherein He freely offers unto sinners life and salvation by Jesus Christ; requiring of them faith in Him, that they may be saved..." (WCF 7.3). In other words, the WCF is teaching that, from the time of Adam, the covenant of grace required faith in Christ for salvation, as Christ is the only Mediator between God and man.
Furthermore, as WCF 8.6 states, echoing Genesis 3: "
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."