They may believe for awhile but then fall away, giving evidence they never belonged to Christ . . .
Note that Hebrews 6:4-8 shows that even true Christians, who have truly repented and become partakers of God's Holy Spirit, can ultimately lose their salvation because of subsequently wrongly employing their free will to "fall away", to commit apostasy, to stop believing (like in Luke 8:13, 1 Timothy 4:1, and 2 Thessalonians 2:3), just as other Bible verses show the same thing (John 15:6; 2 Timothy 2:12b, Mark 8:35-38, Hebrews 10:38-39, Matthew 24:9-13).
One way that a true Christian could be brought to the point where he commits apostasy would be if he finds a particular sin to be very pleasurable, so pleasurable and so fulfilling (in the short term) that he continues in it over time until his heart becomes hardened by the deceitfulness of sin (Hebrews 3:13), to where his love for God grows cold because of the abundance of iniquity (Matthew 24:12), to where he quenches the Spirit (1 Thessalonians 5:19), to where he sears his conscience as with a hot iron (1 Timothy 4:2), to where he begins to listen to the lies of demons and latch onto them, to the point where he departs from the Christian faith (1 Timothy 4:1). In a wrong desire to continue in their lusts without repentance, Christians can reach the point where they are no longer able to endure the sound doctrine of the Bible, and instead seek out and latch onto other teachings which will help to support them in their lusts (2 Timothy 4:3-4).
Another way that a true Christian could be brought to the point where he commits apostasy would be if he has a terror of being tortured and killed during a persecution against Christians, so that during such a persecution he renounces his faith in Jesus Christ and the Gospel to avoid being tortured and killed (Mark 8:35-38; 2 Timothy 2:12). Some Christians will fall away in this sense (2 Thessalonians 2:3) during the future Tribulation of Revelation chapters 6 to 18 and Matthew 24 (Matthew 24:9-13, cf. Matthew 13:21, Luke 8:13), when the future Antichrist will take power over the earth, make war against Biblical Christians (not in hiding), and physically overcome them in every nation (Revelation 13:7-10, Revelation 14:12-13, Revelation 20:4-6, Matthew 24:9-13).
There will be no way to repent from committing apostasy (Hebrews 6:4-8), and worshipping the future Antichrist and his image, and willingly receiving his mark on the forehead or right hand, even if this is done just to keep from getting killed (Revelation 13:15-18). Whoever does these things, even if he had been a Christian before, will end up suffering punishment from God in fire and brimstone forever (Revelation 14:9-12). So Christians must be willing to be killed, even by getting beheaded (Revelation 20:4-6), before they would ever do any of these things (Revelation 14:12-13).
This ties in with the fact that a Christian can ultimately have his name blotted out of the Book of Life if he does not overcome to the end (Revelation 3:5, Revelation 2:26). An example of Christians ultimately "overcoming" (Greek: nikao: G3528), or "getting the victory" (nikao) (Revelation 15:2), is found later in the book of Revelation, in Revelation 15:2, which refers to those Christians who will be willing to be killed by the future Antichrist instead of worshipping him to save their mortal lives during the future, worldwide persecution against Biblical Christians (Revelation 13:7-10, Revelation 14:12-13, Revelation 20:4-6, Matthew 24:9-13). Christians will be able to spiritually "overcome" the Antichrist and Satan by not loving their lives to the death (Revelation 12:11).
