Something to know and understand when looking to learn whether "once saved always saved" is legitimate Christian theology is to remember a few distinctions from the Bible about salvation. That terminology is never stated in scripture so you won't find, "once saved always saved" in scripture. You know this but just for the sake of beginning at the beginning, let it be said.
So to determine if it is legitimate theology you have to ask , "What does it teach/imply?" What does it mean to be saved? When is a person saved? When they receive Jesus Christ as Savior? When they are sanctified? Or on the day of judgement, which we will all see, are we saved at that moment? We've heard in churches all our lives that when you accept Christ as your Savior you are saved from that point on, but does the Bible tell us that? That is a major question. Does the Bible tell us we are then saved? Or does it tell us we are born again? Is there a difference? Yes. We are born again when we become a new creation through God's grace AND our faith in Christ Jesus. Faith without works is dead. So if faith is no more than words, then it is ineffective. You must understand this part of Scripture. James 2:14-22 "What does it profit, my brethren, if someone says he has faith but does not have works? Can faith save him? If a brother or sister is naked and destitute of daily food, and one of you says to them, “Depart in peace, be warmed and filled,” but you do not give them the things which are needed for the body, what does it profit? Thus also faith by itself, if it does not have works, is dead. But someone will say, “You have faith, and I have works.” Show me your faith without your works, and I will show you my faith by my works. You believe that there is one God. You do well. Even the demons believe—and tremble! But do you want to know, O foolish man, that faith without works is dead? Was not Abraham our father justified by works when he offered Isaac his son on the altar? Do you see that faith was working together with his works, and by works faith was made perfect?" See faith without works has no power to save and we are saved by Grace through faith. There is a dual working relationship between God's unmerited favor towards us and our participation in something we can never earn the RIGHTS to. Just because we cannot earn the rights to it does not mean that we don't participate in our salvation. This is why Paul tells BELIEVERS (Philippians 2:12-14) "Wherefore, MY BELOVED, as ye have always obeyed, not as in my presence only, but now much more in my absence, work out your own salvation with fear and trembling. For it is God which worketh in you both to will and to do of his good pleasure." See it is God who works in us to change our desires and to give us the power to do good, BUT in FAITH we have to work out those things willingly. We must allow this to take place in us. If we do not then we blaspheme the Holy Spirit and scripture tells us that is the only unforgivable sin. Mark 3:28-29 tells us all sin and blasphemy can be forgiven, but not the blaspheme of the Holy Spirit. Blaspheme comes from the word blasphēmos which means impious, to act against God, to rail or revile God. To act against the Holy Spirit is to resist the regeneration and sanctification that comes from the Holy Spirit. Simply put, To resist the process of BEING SAVED. Do some research on the word salvation and saved through out scripture and you will find that salvation is always spoken of in the future tense as something that we are now closer to than when we first believed. See Romans 13:11. If we are already saved, how can we be closer today than when we first believed? Because salvation and being born again are different. He who endures to the end SHALL BE SAVED. He who calls upon the name of the Lord SHALL BE saved. Note that the word calls is defined as a continuous reliance on the Lord. It is not a one time walk down the aisle, but a life that is lived day in and day out following Christ. SO, NOW THAT WE HAVE ESTABLISHED THAT WE ARE NOT YET SAVED, BUT SEALED FOR SALVATION AS LONG AS WE CONTINUE WITH THE LORD we see that the concept of once saved always saved only applies to those who have died and gone on to be with the Lord. (those who have come to an end and have endured to the end) Not to us who are still here. Paul spent most of His writings encouraging believers to turn from sin and remain strong in their faith (which, remember, without works is dead) with the Lord. Why would he bother telling believers to live in any specific way if they did not have any ability to "fall away". See Hebrews 6:4-6 "For it is impossible for those who were once enlightened, and have tasted of the heavenly gift, and were made partakers of the Holy Ghost, And have tasted the good word of God, and the powers of the world to come, IF THEY SHALL FALL AWAY, to renew them again unto repentance; seeing they crucify to themselves the Son of God afresh, and put him to an open shame." Also read 1 Timothy 4:1 about falling away and also Matthew 24:10. You cannot fall away from something that has not existed. You cannot be enlightened and not really enlightened at the same time. See 2 Peter 2:20-22 "For if, after they have escaped the pollution of the world through the knowledge of the Lord and Savior Jesus Christ, they are again entangled in them and overcome, the latter end is worse for them than the beginning. For it would have been better for them not to have known the way of righteousness, than having known it, to turn from the holy commandment delivered to them. But it has happened to them according to the true proverb: “A dog returns to his own vomit,” and, “a sow, having washed, to her wallowing in the mire.” We cannot escaped the pollution of the world through Christ and not really have escaped. We have always had and will always have freewill. If we choose to reject the Lord even after we have known Him God will allow us to do so and will give us over to our desires. He doesn't leave or forsake us, our SIN separates us from God and we turn and leave Him and He allows us the freewill to do so. See Isaiah 59:2 This is why sin is so dreadful and I hate it so much. IT is what separates us, not people, not God, but sin itself. Here are a few more verses just for fun to also look up to help gird your reading up on this subject matter. Read the parable of the wedding feast in Matthew chapter 22. Notice that the man comes to the wedding feast and sits at the table with all the rest. He has responded to the call and has entered with all the rest into the wedding, BUT this one came without his wedding garment on. The wedding garment is the fine linen also mentioned in revelations which is being clothed in the good works of faith. What does Jesus say the king did to him? He bound him hand and foot and cast him into the lake of fire. Then we see verse 22:14 ""For many are called, but few are chosen." Everyone is called to the Lord and many come, but do not finish and so are not chosen. Philippians 3:12-14 tells us we have not yet obtains the prize of salvation, that we must press on toward this goal. We can be confident that we will reach this goal, but WE must press on towards it and not cast away our confidence in the Lord. See Hebrews 10:35-36 "Therefore do not cast away your confidence, which has great reward. 36 For you have need of endurance, so that after you have done the will of God, you may receive the promise." Remember when Jesus told the parable of the sower who spread seed (the Gospel) on various types of ground? Remember the thorny ground sprouted and grew, but the thorns choked out the Gospel. Jesus explains that this ground in this parable is talking about those who received the Gospel willingly and started off well, but then the cares of this world choked out the good work God was doing in them. Why? By their choices to care more for what was worldly than what is eternal in Christ; to live lawlessly. See 1 Corinthians 9:24-27 "Know ye not that they which run in a race run all, but ONE receiveth the prize? So run, that ye may obtain. And every man that striveth for the mastery is temperate in all things. Now they do it to obtain a corruptible crown; but we an incorruptible. I therefore SO run, NOT as UNCERTAINLY; so fight I, not as one that beateth the air: But I keep under my body, and bring it into subjection: LEST that by any means, when I have preached to others, I MYSELF should BE a CASTAWAY." If Paul tells us that he maintains his walk lest he be cast away we certainly can be castaway as well. I hope this all helps. It is important to remember that the statement "once saved always saved" is a loaded statement. Only the dead in Christ are saved. :) We are sealed for the day of judgment so long as we do not turn again to the old man and reject the Lord. He who endures to the end shall be saved. I know it is a lot, but this is a really important part of Christian theology. This is in large part most of the new testament teaching. We do not earn salvation by works, we work it out through faith which is seen in works. Earning and working out are different.