I believe that some non-Christians will have a shot in the millennium, that's what its for, a time of teaching and obedience. "Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years" (Revelation 20:6).
We need to differentiate between non-Christians and unbelievers, a non-Christian could be described as one who never heard the gospel and never had a chance to accept Christ. An unbeliever is one who heard the Word but rejected it. I believe the latter will perish, but the former will experience the millennium because they never had an opportunity in the flesh (aborted babies, etc). God is fair., no righteous God would condemn an unknowing person who had no knowledge of Truth; "Father, forgive them; for they know not what they do". Remember too, after his resurrection, Jesus "Went and preached unto the spirits in prison". (1 Peter 3;19) Why? "For this cause was the gospel preached also to them that are dead, that they might be judged according to men in the flesh, but live according to God in the spirit" (1 Peter 4:6).. jmo