"I used to be a Christian."

No, you did not used to be a Christian.  If you become a "born-again" believer, you cannot undo your spiritual birth; anymore than you could undo your physical birth.  You simply deceived yourself for a while and now you finally see that you have never known Christ..

"Yeah, I used be a Christian.  I even taught Sunday School, but I woke up."  Listen friend, sitting in a garage doesn't make you a car!  Likewise, sitting in church doesn't make you a believer. Knowing a couple of Bible stories doesn't make you a believer either; Satan himself can quote the Bible inside and out.  Read James 2:19, "...the devils believe also, and tremble."

You see friend, these people have been DECEIVING themselves.  Satan is a great trickster and he loves what the world calls "religious people."  Dwight L. Moody said, "Most people have just enough religion to be miserable."   They work and work trying to get into God's good graces meanwhile never repenting of their sins and following the Lord Jesus Christ.  The ONLY way to get in God's good graces is through Jesus Christ--Period.  There is simply NO other way.  If you try to enter heaven apart from Jesus Christ alone, you a thief and a robber:

"Verily, verily, I say unto you, He that entereth not by the door into the sheepfold, but climbeth up some other way, the same is a thief and a robber...I am the door." (John 10:1,9)

The religious, unregenerate person is just as hell bound as any other sinner. You see, he or she can live a lie at first, thinking their a Christian; but it is not sustainable in the long run.  Religion can only take you so far my friend.  Notice the following Scriptures...

1 John 2:19, "They went out from us, but they were not of us; for if they had been of us, they would no doubt have continued with us: but they went out, that they might be made manifest that they were not all of us."

1 John 2:15, "Love not the world, neither the things that are in the world. If any man love the world, the love of the Father is not in him."

James 4:4, "Ye adulterers and adulteresses, know ye not that the friendship of the world is enmity with God? whosoever therefore will be A FRIEND OF THE WORLD IS THE ENEMY OF GOD."

John 12:25, "He that loveth his life shall lose it; and he that hateth his life in this world shall keep it unto life eternal."

A Christian's rejection of the world is wholly unnatural; It is supernatural.  I hear folks saying that open nudity is natural, sexual lust is natural, tobacco is natural; on the contrary, God wants us to be spiritual friend, not natural.  The Christian life is a spiritual life.  It is God's Spirit that dwells in our earthly tabernacle and guides us along life's way.  To be spiritual means that we seek the Holy Spirit's guidance and that we yield to His commands.  The unsaved person does NOT know God's Spirit.  We receive God's Holy Spirit the very moment we trust Christ as our Savior.  

1 John 2:22, "Who is a liar but he that denieth that Jesus is the Christ? He is antichrist, that denieth the Father and the Son."

1 John 4:3, "And every spirit that confesseth not that Jesus Christ is come in the flesh is not of God: and this is that spirit of antichrist, whereof ye have heard that it should come; and even now already is it in the world."

2 John 1:7, "For many deceivers are entered into the world, who confess not that Jesus Christ is come in the flesh. This is a deceiver and an antichrist."

So you still say, "I used to be a Christian."  Hmmm!  You see friend, Christianity is not an organization; It is not a denomination; It is not a philosophy; It is not a religion; It is not a belief---Christianity is a person, the Lord Jesus Christ.  Either you accepted the Person of Jesus Christ into your heart and He's still there, or you have never been saved and are in hot water.  If you have tasted of the heavenly gift, but have not received it...what are you waiting for.  One last thought--If you reject Christ as your Savior, you will have forfeited your last and only bus-ticket to heaven and eternal life.  In other words, If you have already heard the truth (the light), but have not acted upon it; there is no more light for you to hear.  If the truth you have heard didn't phase you, then there is nothing that can save you.  

"...If therefore the light that is in thee be darkness, how great is that darkness!" (Matthew 6:23)