"But the Spirit saith expressly, that in later times some shall fall away from the faith..." (I Tim. 4:1). Obviously, giving heed or listening to seducing spirits and the devil's teaching will cause one to fall away. False teachers teach things contrary to God's word and lead others astray.
Other causes include doubt or a simple lack of faith. When we do not take into our hearts and minds the holy word of God, our faith will wane (Rom. l0:17). People become careless and unconcerned about spiritual matters, often because they let the cares of this world choke out the word of God (Mt. 13 :22). The love of money is still the root cause of many sins, including falling away (I Tim. 6: 10).
Association with the wrong crowd can very easily cause some to fall away. Don't be fooled, "evil companionships corrupt good morals" (I Cor. 15:33) because of evil influences, some have been lead to forsake the assembly and to engage in sinful practices. Some simply fall away because they go back to their old ways. Regardless of the sin, reverting to sinful habits of the past will cause some to fall away. Remember Demas loved this present world and forsook the cause and fell away (II Tim. 4:10). "Love not the world..." (I Jn. 2:l5).
Some, like the Laodiceans, become lukewarm. They think they are OK with the Lord when in fact they are about to be spewed out of the mouth of God (Rev. 3216). The Laodiceans were told to repent (Rev. 3:19). Similarly, those who give up too easily, who "become weary in well doing", will likely fall away (Gal. 619). Unfortunately, even crisis or tragedy will sometimes cause one to fall away. They will blame God and turn from him in a time when they most need him.
Whatever the reason, falling away from God will be unacceptable in the day of judgment. Peter urges those who sin to repent and pray (Acts 8:22) Forgive others who repent (Lk. l7:3) so that "your heavenly Father will also forgive you" (Mt. 6:l4). If you have fallen away, please come back to God who waits with open arms (Lk. 15).