A Christian is a person Born-Again by the Spirit of God. Born-Again believers cannot be un-born or detached from their spiritual family in Christ. We are kept eternally secure by the grace of God, not our self-righteousness or our strength. No one can be perfect enough on their own to stand faultless before the most Holy God. There is none righteous, except through the Blood of Christ. Those who believe that they can go to Heaven by following a set of rules are not perfected in grace and are lost.
Often, people who claim to be ex-Christians would have probably just been religious, without understanding the gospel, un-regenerate from the very beginning, not trusted in Christ as Savior in the first place and has nothing to do with the living God.
In a world full of deceptive false religions and false teachers, the true gospel is rarely preached.
Believe in the One True God(Father, Spirit, Son) and that Christ came down to earth as a man, died on the cross and bought our pardon, and is risen on the third day. Accept Christ ALONE(not self-righteousness, not Mary, not Saints, not religion, not sacraments) as your personal saviour.
No Christian will be perfect on this side of Heaven. But we must strive to obey God, out of love for Him and to escape His loving fatherly discipline.
A sinner can be saved from eternal destruction in Hell, by GRACE alone, through FAITH alone, in CHRIST alone.
We need Christ's perfect righteousness imputed in us.
Jesus is The Only Way to Heaven.
John 3:16 - For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.
Romans 6:23 - For the wages of sin is death, but the gift of God [is] eternal life through Jesus Christ our Lord.