For many years people have debated the Biblical doctrine of Security. Some have called it “once in grace, always in grace”, others “once saved, always saved”. However it is referred to, the doctrine of “The Security of the Believer” is the teaching that when a person accepts the Grace of God (i.e., by repentance and faith in the shed blood of Jesus Christ), that person experiences a re-birth. The ‘old’ man dies (as far as the spirit is concerned) and is reborn as a new creation in Christ. The Holy Spirit seals this re-born spirit as a pledge or guarantee that the redemption that has begun in that person WILL come to fruition. This is SECURITY. God cannot lie. He does not lie. He will not lie. Ephesians 4:30 states, “And grieve not the holy Spirit of God, whereby ye are sealed unto the day of redemption.”
Now whether one translates the word "eis" concretely with emphasis on direction and motion, with reference to time and duration, or in the abstract as “with reference to”, the ‘sealing’ points to ‘the day of redemption’. This is when Jesus returns and believers receive their glorified bodies (full redemption). Of course, when we become like Jesus, glorified body and all, we will not have to worry about a ‘sin’ nature for all eternity. But what about between the ‘day’ one believes and the ‘day’ of redemption? If this soul should decide to “jump out” of the hand of God, rebel and forsake his “Christian” walk, then what happens to the SEAL OF THE HOLY SPIRIT. The idea behind this SEAL is an impress for privacy, secrecy, security, and authority. Kings sealed with a signet scrolls, letters, writings that were not to be read except at the appointed time and by the appointed individual. Daniel was told, “Go thy way, Daniel: for the words are closed up and sealed till the time of the end.” That means ‘forget it boys’, you’re not going to find out “till the time of the end.”
The point is simply this: The seal of the Holy Spirit (which protects the spirit of man) guarantees the safe and secure delivery of that soul to the day of redemption (of that soul). If, for ANY reason, that soul is not brought unto “the day of redemption”, then either the seal (Holy Spirit of God) has been broken (overpowered by a greater power -- which does not exist), or God simply lied. Neither is the case. Thus salvation and security are inseparable. One cannot have one without the other. If you are genuinely saved, then you are secure. And if you have not security, then you have not salvation.
If one is not ETERNALLY saved, then he is not saved at all.