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.” (The New John Gill Exposition of the Entire Bible)
So John Gill taught that the Calvinistically elect were in Christ from before the foundation of the world. The strange thing is that if they were eternally in Christ, then how did they ever become in Adam? Surely, no one can exist simultaneously in both.
Here is a link to a Blog discussion on this topic.