When people come to know Christ as their Savior, they are brought into a relationship with God that guarantees their salvation as eternally secure. To be clear, salvation is more than saying a prayer or “making a decision” for Christ; salvation is a sovereign act of God whereby an unregenerate sinner is washed, renewed, and born again by the Holy Spirit (John 3:3; Titus 3:5). When salvation occurs, God gives the forgiven sinner a new heart and puts a new spirit within him (Ezekiel 36:26). The Spirit will cause the saved person to walk in obedience to God’s Word (Ezekiel 36:26–27; James 2:26). Numerous passages of Scripture declare the fact that, as an act of God, salvation is secure:
(a) "Moreover whom He did predestinate, them He also called: and whom He called, them He also justified: and whom He justified, them He also glorified." Romans 8:30. This verse spells out, that from the moment God chooses a person, it is as if they are glorified in His presence in Heaven. There is nothing that can prevent a believer from one day being glorified because God has already purposed it in heaven. Once a person is justified, his salvation is guaranteed—he is as secure as if he is already glorified in Heaven.
“And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand. My Father, which gave them me, is greater than all; and no man is able to pluck them out of my Father's hand.” John 10:28-29
(b) Paul asks two crucial questions.. "Who shall lay any thing to the charge of God's elect? It is God that justifieth. Who is he that condemneth? It is Christ that died, yea rather, that is risen again, who is even at the right hand of God, who also maketh intercession for us." Romans 8:33-34
Who will bring a charge against God’s elect? No one will, because Christ is the advocate for those who accept Him. Who will condemn Christians? No one will, because Christ, the One who died for us, is the one who condemns. We have both the advocate and judge as Savior.
(c) Believers are born again (regenerated) when they believe (John 3:3; Titus 3:5). For a Christian to lose his salvation, he would have to be un-regenerated. The Bible gives no evidence that the new birth can be taken away.
(d) The Holy Spirit indwells all believers (John 14:17; Romans 8:9) and baptizes all believers into the Body of Christ “For by one Spirit are we all baptized into one body, whether we be Jews or Gentiles, whether we be bond or free; and have been all made to drink into one Spirit.” 1 Corinthians 12:13. For a believer to become unsaved, he would have to be "un-indwelt" and detached from the Body of Christ.
(e) John 3:15 states that whoever believes in Jesus Christ will "have eternal life." If you believe in Christ today and have eternal life, but lose it tomorrow, then it was never "eternal" at all. Hence, if you lose your salvation, the promises of eternal life in the Bible would be in error.
(f) When a person is saved, their heart is changed towards God, they hear and ultimately respond to the conviction of sin by the Holy Spirit. It is not to say that one won’t make mistakes after being saved, it is to say that the Christian is forgiven and desires to do that which is right, a tangible testament to the new nature or new creature one becomes.
“Therefore if any man be in Christ, he is a new creature: old things are passed away; behold, all things are become new.” 2 Corinthians 5:17
The conclusive argument: "For I am persuaded, that neither death, nor life, nor angels, nor principalities, nor powers, nor things present, nor things to come, Nor height, nor depth, nor any other creature, shall be able to separate us from the love of God, which is in Christ Jesus our Lord." Romans 8:38–39
The same God who saved you is the same God who will keep you. Once we are saved, we are always saved. Our salvation is most definitely eternally secure!