The Greek words translated “to apostatize” (lit. to stand away, aphistemi) and “apostasy” (defection, apostasia) do not indicate in themselves whether believers or unbelievers are in view. The reader must determine this by the context. Here believers seem to be in view (as in Luke 8:13; Acts 15:38; 1 Tim. 4:1; cf. Luke 2:38; 2 Tim. 2:12b; 4:4) since the writer called them “brethren.” In other contexts, unsaved apostates are in view (e.g., Luke 13:27; cf. 2 Thess. 2:11). In still other passages there is not sufficient data to pass judgment on their salvation (e.g., 2 Thess. 2:3; cf. Titus 1:14). Other Scripture seems to reveal quite clearly that genuine Christians can renounce their faith (Matt. 10:33; Mark 8:32; 2 Tim. 2:12; Rev. 3:8). However this does not mean they will lose their salvation (John 10:28; 2 Tim. 2:13).
Constable, Tom. Tom Constable’s Expository Notes on the Bible. Galaxie Software, 2003. Print.