"I, the LORD, do not change." (Malachi 3:6)
The faith by which we are to walk is grounded in the character of the One who changed us. Our faith rests primarily in the truth that He never changes. His immutability is the bedrock upon which our confidence and assurance of salvation are built.
His constancy is the over-arching attribute which encompasses them all. For, could we depend in faith upon a god whose character vacillates? What would become of all His promises, His declarations and determinations if He could change at any moment? How could we live in joyful, confident expectation if the god we served might keep His word?
Yet, there are multiplied millions of professing (and genuine, as well) believers who assert and teach that it is possible to lose one's salvation. It is contended that the Christian can commit sin so heinous as to forfeit the salvation he once experienced and enjoyed.
The implications of such a position are staggering: (a) the character of the immutable God is impugned, casting doubt upon His precious and magnificent promises related to eternal security; (b) the atoning work of the Lord Jesus Christ is maligned, considered insufficient to completely cover the believer's sins for eternity; (d) the ministry of the blessed Holy Spirit is demeaned, implying that His sealing, sanctifying, and securing work can, at the last, be overcome by sin.
(e) Such a perspective also shifts the basis for salvation from grace to works(!) The question immediately comes: How many good works are necessary to keep one saved, if, indeed, salvation can be lost?
No, salvation is from the LORD (Jonah 2:9), first to last, and He CANNOT be thwarted.
The one whom the Father sets His heart upon from all eternity in election, whose sin is atoned for by the Son, and regenerated by the Holy Spirit will persevere in the faith to the end of his days.