Though many today believe in “the age to come” (Lk. 18:30)—i.e. “the restoration of all things” (Acts 3:21), “the resurrection of the dead” (1 Cor. 15:21), “a new heavens and new earth” (2 Pe. 3:13), etc.—instead of the eternal existence of the soul in an immaterial heaven, not many believe in a transitional messianic kingdom which initiates the age to come. However, the early church clearly believed in a 1000 year transition before the “unending ages” (i.e. eternity) of perfection. But where did they get this 1000 year belief? Most just assume it comes from a literalist reading of Rev. 20:1-6, cf. “they came to life and reigned with Christ for a thousand years” (v. 4). Though this passage stands on its own exegetically, it is not the basis of millennialism in the early church.
The early church believed in what is known as the “cosmic week” or “creation-day, world-age” theory. Each day of creation represents 1000 years of redemptive history. This idea developed primarily out of the interpretation of Gen. 2:17, “for in the day that you eat of it you shall surely die.” Thus, Adam died within 1000 years because “with the Lord one day is as a thousand years” (2 Pe. 3:8; cf. Ps. 90:4). This is how the death of Adam and the age to come were interpreted in early Jewish literature (cf. Jubilees 4:29f; 2 Enoch 32.1-33.1; Life of Adam and Eve, 51.1f), and it is found throughout the writings of the early church (cf. Epistle of Barnabas, 15.3f; Justin Martyr, Dialogue with Trypho, 80-81; Irenaeus, Against Heresies, 5.28.3; Methodius, Extracts, 9; Commodian, Instructions, 35, 80; Lactantius, Epitome of the Divine Institutes, 70). Though this nifty little equation might sound rather quaint to the modern ear, it still proves to be a superior interpretation of Gen. 2:17 and a superior foundation than Rev. 20.