"Once Saved - Always Saved" and is a logical application of John Calvin's teaching of soteriological determinism and absolute double predestination. Calvin taught that once we come to faith, we cannot NOT believe, we're locked into faith. He taught that we have NO free will of any nature or type before we believe - and NO free will after we believe, it's all just a matter of fate. Some Calvinists continue this doctrine today. The great majority of Christians disagree.
Here are MY thoughts...
There is a Law/Gospel dynamic here. The "answer" is not in trying to force the two to 'fit' together, but in their appropriate application.
IF a Christian was overwhelmed by their unworthiness and weakness, if they were overwhelmed by the power of sin and Satan, if they were truly concerned that they might not be saved, here's what I'd say:
For those God foreknew he also predestined to be conformed to the likeness of his Son, that he might be the firstborn among many brothers. And those he predestined, he also called; those he called, he also justified; those he justified, he also glorified. What, then, shall we say in response to this? If God is for us, who can be against us? He who did not spare his own Son, but gave him up for us all--how will he not also, along with him, graciously give us all things? Who will bring any charge against those whom God has chosen? It is God who justifies. Who is he that condemns? Christ Jesus, who died--more than that, who was raised to life--is at the right hand of God and is also interceding for us. Who shall separate us from the love of Christ? Shall trouble or hardship or persecution or famine or nakedness or danger or sword? As it is written: "For your sake we face death all day long; we are considered as sheep to be slaughtered." No, in all these things we are more than conquerors through him who loved us. For I am convinced that neither death nor life, neither angels nor demons, neither the present nor the future, nor any powers, neither height nor depth, nor anything else in all creation, will be able to separate us from the love of God that is in Christ Jesus our Lord. Romans 8:29-39
"For God so loved the world that he gave his one and only Son, that whoever believes in him shall not perish but have eternal life. " John 3:16
"For false Christs and false prophets will appear and perform signs and miracles to deceive the elect--if that were possible. Mark 13:22
"but whoever drinks the water I give him will never thirst. Indeed, the water I give him will become in him a spring of water welling up to eternal life." John 4:14
"I give them eternal life, and they shall never perish; no one can snatch them out of my hand. John 20:28
"The one who calls you is faithful and he will do it. 1 Thess. 5:24
"because by one sacrifice he has made perfect forever those who are being made holy. Hebrews 10:14
"I will never blot out his name from the book of life, but will acknowledge his name before my Father and his angels." Rev. 3:5
I would NOT make doctrine out of these, but I would apply these - and others - to comfort and assure them.
IF the person thought, "Hey, why not eat, drink and be merry? Hey, I can do whatever I want, think whatever I want, believe whatever I want - because 'ONCE SAVED, ALWAYS SAVED!' Hey, I was baptized, hey I did the Billy Graham altar call thing, so I'm saved- NO MATTER WHAT!" Here's what I'd say:
"Remain in me, and I will remain in you... If anyone does not remain in me, he is like a branch that is thrown away and withers; such branches are picked up, thrown into the fire and burned." John 15:4-7
"Be faithful, even to the point of death, and I will give you the crown of life. Rev. 2:10
"He who stands firm to the end will be saved." Matthew 10:22
"The Spirit clearly says that in later times some will abandon the faith and follow deceiving spirits and things taught by demons." 1 Tim. 4:1
"They believe for a while, but in the time of testing they fall away." Luke 8:13
"If you hold to my teaching, you are really My disciples." John 8:31
"By standing firm you will gain life." Luke 21:19
"They did not remain faithful to My covenant, and I turned away from them" Hebrews 8:9
"You who are trying to be justified by law have been alienated from Christ; you have fallen away from grace." Gal. 5:4
"If you continue in your faith, established and firm, not moved from the hope held out in the gospel." Col. 1:23
"If we deliberately keep on sinning after we have received the knowledge of the truth, no sacrifice for sins is left, but only a fearful expectation of judgment and of raging fire that will consume the enemies of God." Hebrews 10:26
"But if anyone does not have them, he is nearsighted and blind, and has forgotten that he has been cleansed from his past sins. Therefore, my brothers, be all the more eager to make your calling and election sure." 2 Peter 1:8-10
"Be on your guard so that you may not be carried away by the error of lawless men and fall from your secure position." 2 Peter 3:17
He who overcomes will, like them, be dressed in white. Rev. 3:5
"He who disowns Me before men will be disowned before the angels of God." Luke 12:8
And all the many verses that talk about being faithful... I would NOT make doctrine out of this, it's just the verses I might apply in such a case.
As so often in theology, we find there are "two sides of the coin." To ME, the approach is NOT to take all the Scriptures, subject them to our limited, fallable, sinful, human LOGIC and force them to "fit" and "make sense" to US. To ME, the approach is to accept both "sets" of scriptures at their face value and allow them to stand just as God inspired them. The approach, then, is in how to APPLY them rather than in how to force them to fit together.
MY view. What's YOURS?
Keep the faith! Share the love! - Josiah