You think anyone who falls away is not an unbeliever? What is your understanding of this passage:
2 Thess 2:1 Now we beseech you, brethren, by the coming of our Lord Jesus Christ, and by our gathering together unto him, 2 That ye be not soon shaken in mind, or be troubled, neither by spirit, nor by word, nor by letter as from us, as that the day of Christ is at hand. 3 Let no man deceive you by any means: for that day shall not come, except there come a falling away first, and that man of sin be revealed, the son of perdition;
What do you think Paul is referring to when he talks about "a falling away" here? Are those who fall away still true believers after falling away?
Are you somehow not aware that you and I and all other believers will also will be judged by our works?
Romans 14:10 But why dost thou judge thy brother? or why dost thou set at nought thy brother? for we shall all stand before the judgment seat of Christ. 11 For it is written, As I live, saith the Lord, every knee shall bow to me, and every tongue shall confess to God. 12 So then every one of us shall give account of himself to God.
2 Corinthians 5:10 For we must all appear before the judgment seat of Christ; that every one may receive the things done in his body, according to that he hath done, whether it be good or bad.
Revelation 22:12 And, behold, I come quickly; and my reward is with me, to give every man according as his work shall be.
Look at Matthew 25:31-46, also.