The factor that makes one a believer, and therefore saved, is FAITH! Very simply, when someone has the faith to believe, and then they confess Jesus Christ as Lord, they are saved! The Just shall live BY faith!

For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Eph. 2:8

That if thou shalt confess with thy mouth the Lord Jesus, and shalt believe in thine heart that God hath raised him from the dead, thou shalt be saved. [10] For with the heart man believeth unto righteousness; and with the mouth confession is made unto salvation. Romans 10:9-10

For therein is the righteousness of God revealed from faith to faith: as it is written, The just shall live by faith. Romans 1:17

But to him that worketh not, but believeth on him that justifieth the ungodly, his faith is counted for righteousness. Romans 4:5

For what saith the scripture? Abraham believed God, and it was counted unto him for righteousness. Romans 4:3

Cometh this blessedness then upon the circumcision only, or upon the uncircumcision also? for we say that faith was reckoned to Abraham for righteousness. Romans 4:9

Holding faith, and a good conscience; which some having put away concerning faith have made shipwreck: 1 Tim. 1:19

Who concerning the truth have erred, saying that the resurrection is past already; and overthrow the faith of some. 2 Tim. 2:18

Now as Jannes and Jambres withstood Moses, so do these also resist the truth: men of corrupt minds, reprobate concerning the faith. 2 Tim. 3:8

Now the Spirit speaketh expressly, that in the latter times some shall depart from the faith, giving heed to seducing spirits, and doctrines of devils; 1 Tim. 4:1

But the younger widows refuse: for when they have begun to wax wanton against Christ, they will marry; [12] Having damnation, because they have cast off their first faith. 1 Tim. 5:11-12

If thou put the brethren in remembrance of these things, thou shalt be a good minister of Jesus Christ, nourished up in the words of faith and of good doctrine, whereunto thou hast attained. 1 Tim. 4:6

The Galatians, by trusting in the law to save them, instead of Faith, had "fallen from grace". Grace IS ONLY given us through our FAITH! Without FAITH, there is NO GRACE! If we lose faith, we fall from Grace!

Christ is become of no effect unto you, whosoever of you are justified by the law; ye are fallen from grace. Galatians 5:4

For by grace are ye saved through faith; and that not of yourselves: it is the gift of God: Eph. 2:8

The gift of the Holy Spirit came by the hearing of faith, and when the Galatian church began to trust their works instead of faith to save them, Paul rebuked them!

This only would I learn of you, Received ye the Spirit by the works of the law, or by the hearing of faith? [3] Are ye so foolish? having begun in the Spirit, are ye now made perfect by the flesh? Galatians 3:2-3

Paul charges the Colossian church to "continue in the faith" as a condition by which Christ will "present you holy and unblameable and unreproveable in his sight"

And you, that were sometime alienated and enemies in your mind by wicked works, yet now hath he reconciled [22] In the body of his flesh through death, to present you holy and unblameable and unreproveable in his sight: [23] If ye continue in the faith grounded and settled, and be not moved away from the hope of the gospel, which ye have heard, and which was preached to every creature which is under heaven; whereof I Paul am made a minister; Col. 1:21-23

Paul's great concern with the Thessalonian church, was to find if they were still in the Faith!

For this cause, when I could no longer forbear, I sent to know your faith, lest by some means the tempter have tempted you, and our labour be in vain. 1 Thes. 3:5

There were those who "erred from the faith", but Paul warns us to "fight the good fight of faith" by which we can "lay hold (seize) eternal life!

For the love of money is the root of all evil: which while some coveted after, they have erred from the faith, and pierced themselves through with many sorrows. [11] But thou, O man of God, flee these things; and follow after righteousness, godliness, faith, love, patience, meekness. [12] Fight the good fight of faith, lay hold on eternal life, whereunto thou art also called, and hast professed a good profession before many witnesses. 1 Tim. 6:10-12

I have fought a good fight, I have finished my course, I have kept the faith: 2 Tim. 4:7

If a man shall "draw back" from the faith, God will have "no pleasure in him" and the person will be destroyed (perdition). We MUST continue to believe in order to save our soul! Chapter 11 gives us the "cloud of witnesses' who continued to believe and their faith did not disapoint them.

Now the just shall live by faith: but if any man draw back, my soul shall have no pleasure in him. [39] But we are not of them who draw back unto perdition; but of them that believe to the saving of the soul. Hebrews 10:38-39

I know thy works, and where thou dwellest, even where Satan's seat is: and thou holdest fast my name, and hast not denied my faith, even in those days wherein Antipas was my faithful martyr, who was slain among you, where Satan dwelleth. Rev. 2:13

But whosoever shall deny me before men, him will I also deny before my Father which is in heaven. Matthew 10:33

If we suffer, we shall also reign with him: if we deny him, he also will deny us: 2 Tim. 2:12

There will be those who will only endure as long as they don't have to suffer for being a Christian.

And these are they likewise which are sown on stony ground; who, when they have heard the word, immediately receive it with gladness; [17] And have no root in themselves, and so endure but for a time: afterward, when affliction or persecution ariseth for the word's sake, immediately they are offended. Mark 4:16-17

The word for offend is: skandalizo, skan-dal-id'-zo; (“scandalize”); from Greek 4625 (skandalon); to entrap, i.e. trip up (figurative stumble [transitive] or entice to sin, apostasy or displeasure) :- (make to) offend.

Scripture clearly indicates that, before Jesus returns, there will be believers who will fall away from the faith, "apostasy"! These people are NO LONGER saved!

Let no man deceive you by any means: for that day shall not come, except there come a falling away first (apostasy), and that man of sin be revealed, the son of perdition; 2 Thes. 2:3

apostasia, ap-os-tas-ee'-ah; feminine of the same as Greek 647 (apostasion); defection from truth (properly the state) ["apostasy"] :- falling away, forsake.

The offended brother goes back to his sin, and apostatizes from the faith! Their love for God grows cold! But, be warned, only those who endure to the end that will be saved!

If your faith is weak, your in dangerous territory! If you have lost your faith, and you can no longer confess Jesus as your Lord, your LOST and perishing! If you have the faith to believe, confess to Jesus that HE is Lord, and surrender to Him!

Wherefore I give you to understand, that no man speaking by the Spirit of God calleth Jesus accursed: and that no man can say that Jesus is the Lord, but by the Holy Ghost. 1 Cor. 12:3

We having the same spirit of faith, according as it is written, I believed, and therefore have I spoken; we also believe, and therefore speak; 2 Cor. 4:13

It is when our faith is weak, that we begin to trust in our flesh, instead of trusting God, and then we sin!

But every man is tempted, when he is drawn away of his own lust, and enticed. [15] Then when lust hath conceived, it bringeth forth sin: and sin, when it is finished, bringeth forth death. James 1:14-15

Brethren, if any of you do err from the truth, and one convert him; [20] Let him know, that he which converteth the sinner from the error of his way shall save a soul from death, and shall hide a multitude of sins. James 5:19-20