Q: As I understand your premise in Grace in Focus, you believe that if a person makes a sincere profession of faith but later became an apostate that he/she is still saved. If that is true, will there be non-believers in heaven? ~D.T.
A: So, to clarify—a sincere profession of faith is not enough to be saved. A profession is simply a claim to believe. You are saved through believing in Jesus, not by professing to believe in Jesus.
With that said, anyone who believes in Jesus for eternal life has it forever, even if he or she becomes an atheist. All it takes is a single act of faith, not a lifetime of faith. And once you believe, you cannot lose your eternal life, or regeneration, or justification, etc., no matter what you do or fail to do. So, in that sense, a current atheist will go to be with the Lord when he dies if he has ever believed in Jesus for everlasting salvation—once saved, always saved.
But that does not mean that apostasy is consequence-free. For example, when he appears before the Judgment Seat of Christ to receive his eternal rewards (1 Cor 3:11-15; 2 Cor 5:10), he will feel great shame for his atheism and suffer a loss because of it. Of course, at that point, he will also no longer be an atheist. So, in that sense, there are no atheists in heaven.