This is true and remains a steady constant from the time Adam and Eve were chased from the garden to the eternal state of the new heaven and new earth. Because of mankind’s depraved fallen condition inherited from the first man, if anyone was to be saved, God had to do it Himself. This means Old Testament saints, New Testament Christians, and those who will be converted during the future millennium, are all saved by the choice of God (election) and the workmanship of God (grace). If it is eternal life that is given, then it is God entirely responsible, for only God’s work can be eternal. Although there are differences in the final place and position God’s redemption brings about for different groups being saved, and these differences by the choice of God and seen in Scripture dependent on dispensation, still God’s work of quickening souls (giving them eternal life) has been going on at least from the time of Abel (John 5:17-21).
Now God in His grace has set us before Him without unanswered questions or ongoing issues: they have been all solved in Jesus Christ and God. However, we will never be at peace until we have fully comprehended ourselves under the efficacy of all that Jesus Christ has done for our salvation, and we cannot enjoy it as long as there are questions to be solved.
Articles that discuss this truth in more detail: