Passages which record cases of actual apostasy, 1 Timothy 1:19,20; 2 Timothy 2:17, 18; 4:10; 2 Peter 2:1,2. But there is no proof that the persons mentioned were true believers. The Bible itself teaches that there are persons who profess the faith and yet are not of the faith, Romans 9:6; 1 John 2:9, Revelation 3:1. John says of some: "They went out from us, but they were not of us; for if they had been of us, they would have continued with us," 1 John 2:19.
Louis Berkhof, Manual of Christian Doctrine, Eerdmans, p. 276.