Jesus Christ's return will wrench open a great divide. This time of rapture will divide humanity into two distinct groups which you might label "The Haves" and "The Have Nots." The "Haves" are the believers in Christ and the "Have Nots" will not believe in Christ—Yet!
The 1,000-year reign of Christ, called the Millennium, will offer those who have not known Christ an opportunity to believe in Him. This is not an "extra chance," it is the endmost and only remaining opportunity for the unbelievers. Just think back over your own life, how many times could Jesus and the Holy Spirit have given up on you?