The Sabbath is a model and an image of the coming kingdom of the saints, when the saints shall co-reign with Christ, when he arrives from heaven, as also John in his Apocalypse describes. For a day of the Lord is as a thousand years. And so since in six days God made all things, it is necessary for six thousand years to be fulfilled.

-Hippolytus, Commentary on Daniel Book 2, 23.5-6