Just as it is for us today, while living in the Ephesians 3:2 age of God's grace, it was by Abraham having faith that in God's eyes imputed righteousness was worthy of being gifted to Abraham. Abraham simply believed God and that was sufficient.
There is no reason to believe that members of the church, the body of Christ, would have an eternal gift of God taken away.
Ephesians 4:30
And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.
Ephesians 2:8-9
For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: 9 Not of works, lest any man should boast.
If it is by God's grace that we are gifted salvation, and not by anything of our own doing, why would God take back His gift? That would imply that His gift (or retaining of it) was actually based on our meriting it, rather than simply offered to us because of His grace (love). Therefore, we are gifted salvation (and the imputed righteousness that comes with it) upon having faith alone in Him alone once and for all of eternity.
Ephesians 1:12-13
That we should be to the praise of his glory, who first trusted in Christ. 13 In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, ye were sealed with that holy Spirit of promise,