When does a person technically become "in Christ"?
A) Before the foundation of the world, certain "elect" people are already "in Christ," not that they are chosen to become in Christ, but already are in Christ, from eternity past. As a result, when they are born, they already are "in Christ," without even knowing it, until such time as Irresistible Grace makes them aware of it.
B) Certain "elect" people are born "in Adam," but at an appointed time, are preemptively made Born Again "in Christ," regenerated with a new heart and a new spirit as the "new creture" in Christ, and thus are efficaciously drawn to Christ.
C) A person is born "in Adam," and does not become "in Christ" until he hears and believes in the Gospel, and then is "sealed" in Christ, as per Ephesians 1:13.
I believe that Calvinist, James White, taught (B):
James White: “When the time comes in God’s sovereign providence to bring to spiritual life each of those for whom Christ died, the Spirit of God will not only effectively accomplish that work of regeneration but that new creature in Christ will, unfailingly, believe in Jesus Christ (‘all that the Father gives Me will come to Me’). Hence, we are not saved ‘without’ faith, but at the same time, Christ’s atonement is not rendered useless and vain without the addition of libertarian free will.” (Debating Calvinism, p.191)
Jacob Arminius taught (C):
Arminius: “God regards no one in Christ unless they are engrafted in him by faith.”
I personally believe that Paul taught (C) as well:
Paul: “In Him, you also, after listening to the message of truth, the gospel of your salvation--having also believed, you were sealed in Him with the Holy Spirit of promise.” (Ephesians 1:13)
However, I couldn't find a single major Calvinist who was willing to espouse (A), until now.
In his commentary on 2nd Timothy 1:9, John Gill states: "which was given us in Christ Jesus before the world began; it is a gift, and a free gift, not at all depending upon any conditions in the creature, and entirely proceeding from the sovereign will of God; and it was a gift from eternity; there was not only a purpose of grace in God's heart, and a promise of it so early, but there was a real donation of it in eternity: and though those to whom it was given did not then personally exist, yet Christ did, and he existed as a covenant head and representative of his people; and they were in him, as members of him, as represented by him, being united to him; and this grace was given to him for them, and to them in him; in whom they were chosen, and in whom they were blessed with all spiritual blessings. The Ethiopic version reads, "in Christ Jesus, who before the world [was]"; but without any foundation." (http://www.studylight.org/com/geb/view.cgi?book=2ti&chapter=1&verse=9)
Now the interesting thing is, if they were "in him," that is, Christ, from eternity past, "as members of him," then compare with 1st John 2:23-24: "Whoever denies the Son does not have the Father; the one who confesses the Son has the Father also. As for you, let that abide in you which you heard from the beginning. If what you heard from the beginning abides in you, you also will abide in the Son and in the Father."
So if a person could be "in the Son" from before the foundation of the world, doesn't it stand to reason that they were "in the Father" from before the foundation of the world, also?