Eternal Security is the teaching that once you are truly saved, you can’t lose your salvation. This teaching is also referred to as the “Perseverance of the Saints” — the ‘P’ in Calvin’s TULIP — or “Once Saved-Always Saved”.
Some that teach this doctrine would hold if you gave your heart to Jesus at an early age and then later turned from God, you would still go to heaven when you die: Even if you lived a wicked and sinful life and didn’t even believe there was a God.
Others take a more conservative view and would hold that while you can’t lose your salvation, if you forsook Christ and never came back to him you were never saved in the first place.
But regardless of which position of Eternal Security is taken, all Eternal Security teachers hold that once you are truly saved, you can’t spiritually die.
The Bible refers to spiritual death in a number of scriptures, and death in these scriptures refers to spiritual separation from God. I have covered this in an article Spiritual Death, which you may want to refer to.
I would hold that the Bible teaches that if a Christian willfully sins, they will spiritually die — sin will cause separation from God. If this sin is not repented of, and the person dies in this state, they will be lost.
Now to the title of this article — who was the First Eternal Security Teacher in the Bible? In Geneses 3:3 we find the serpent teaching this very doctrine to Eve. “And the serpent said unto the woman, ye shall not surely die.”
Satan told Eve that she could sin and that she would not experience spiritual death or spiritual separation from God — “Ye shall not surely die.”
This was a lie, as God had said, “for in the day that thou eatest thereof thou shalt surely die” (Gen 2:17); and God’s word didn’t fail. Eve ate and “in the day” she ate, she died.
If you have been told that a Christian can sin and not experience spiritual death, don’t believe it.