God's grace means that we don't have to earn our salvation, we don't have to worry our sins are too great, and we don't have to live the Christian life by our own effort.
Premillennialism believes in Christ's physical return before a literal 1,000-year earthly reign, with historic premillennialism placing it after the tribulation and dispensational premillennialism holding a pre-tribulation rapture of the church.
Postmillennialism is a Christian belief that Christ will return after a period of Christian prosperity in which the church's influence will transform society through the gospel.