The Heresy of OSAS (Once Saved Always Saved)

This heresy is the belief that once you become "Saved" you cannot lose your salvation, regardless of how many sins you commit or their seriousness. This heresy originated with John Calvin (founder of Calvinism) in the mid-sixteenth century. Even Martin Luther did not agree with John Calvin's "theory". Prior to Calvin, the unanimous consent of the early Christians was that a person is capable of losing his salvation by committing mortal sin, as John spoke about in 1 John 5:16–17. Justification of this heretical doctrine is often attempted by way of John 10:28.

John 10:28
And I give them life everlasting: and they shall not perish for ever. And no man shall pluck them out of my hand.

This verse does not say God cannot take away your salvation.

"In the case of John 10:28, Jesus says that no one will be able to take us away from God. The language is similar to Paul’s in Romans 8:39 when he says that nothing in creation will be able to separate us from the love of God in Christ Jesus. Both of these passages address the same fact that no one is capable of removing you from the grace of God. No one is capable of nullifying your salvation. It would be like saying that no one is capable of pulling you out of a car driving at eighty miles per hour. This does not mean that you are incapable of opening the door and jumping out. In the same way, John 10:28 does not mean that we are incapable of severing our relationship with God. Read on in John, and you’ll see why."

(Catholic Answers: Once Saved Always Saved).

Revelation 22:19
And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from these things that are written in this book.

• Once your name is "the book of life" it can be stricken by God if you commit this sin.
  
• The punishment is clear.

Matthew 12:31
Therefore I say to you: Every sin and blasphemy shall be forgiven men, but the blasphemy of the Spirit shall not be forgiven.

This verse tells of another sin that will not be forgiven men. Blasphemy of the Holy Spirit. "Saved" or not, you will not be forgiven this.

1 John 5:16
If any one sees his brother committing what is not a mortal sin, he will ask, and God will give him life for those whose sin is not mortal. There is sin which is mortal; I do not say that one is to pray for that.

1 John 5:17
All wrongdoing is sin, but there is sin which is not mortal.

Luke 6:46
And why call you me, Lord, Lord; and do not the things which I say?

Matthew 7:21
Not every one that saith to me, Lord, Lord, shall enter into the kingdom of heaven: but he that doth the will of my Father who is in heaven, he shall enter into the kingdom of heaven.

See also:
Apostolic Apologetics: Mortal Sin
Catholic Answers: Once Saved Always Saved?

Presented by Apostolic Apologetics