The Bible says in the last days, many will fall away. Many are Christians.

” Now, brethren, concerning the coming of our Lord Jesus Christ and our gathering together to Him, we ask you, not to be soon shaken in mind or troubled, either by spirit or by word or by letter, as if from us, as though the day of Christ had come. Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of sin is revealed, the son of perdition, who opposes and exalts himself above all that is called God or that is worshiped, so that he sits as God in the temple of God, showing himself that he is God.” 2 Thess 2:1-4

What is the cause? Rebellion. A rebellion against the truth of God’s word. Jesus warns us to “watch out that no one deceives us (Mathew 24:4).