This is long. Feel free to not read my reply. These topics are too complex for simple/quick answers.
Once Saved Always Saved? I do not like the term. It is too simplistic to be useful.
What does it mean to be saved? John 3 has the best answers to this question.
John 3:13-15
No one has ascended into heaven except he who descended from heaven, the Son of Man. 14 And as Moses lifted up the serpent in the wilderness, so must the Son of Man be lifted up, 15 that whoever believes in him may have eternal life.The quick answer to "what is salvation" is eternal life with the Trinity in Heaven for all time. Some might call it eternal Sabbath rest.
I do believe in an "Already and Not Yet" aspect to salvation
Romans 8:28-30 states that
And we know that for those who love God all things work together for good] for those who are called according to his purpose. 29 For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers. 30 And those whom he predestined he also called, and those whom he called he also justified, and those whom he justified he also glorified.Some people call those verses the "Golden Chain of Redemption" Notice the past tense. God's predestination, calling, justification, and glorification of His people happens in eternity. It is not something that He will go back on.
I do believe in the surety of salvation for the truly saved. Of course, there are a ton of people who make confessions of faith that are not truly saved. I will come back to that.
The Bible makes salvation a guarantee for the true confessor.
Romans 10:9-10
because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. 10 For with the heart one believes and is justified, and with the mouth one confesses and is saved.Galatians is a great resource on this topic. Paul details how when someone is saved they are "crucified with Christ". That isn't something that can be reversed. Once someone is saved they have Christ living in them.
Galatians 2:19-21 For through the law I died to the law, so that I might live to God. 20 I have been crucified with Christ. It is no longer I who live, but Christ who lives in me. And the life I now live in the flesh I live by faith in the Son of God, who loved me and gave himself for me. 21 I do not nullify the grace of God, for if righteousness were through the law, then Christ died for no purpose.Ephesians 1:13-14 In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit, 14 who is the guarantee[d] of our inheritance until we acquire possession of it, to the praise of his glory.Not only does the believer get Christ living in him, but he also gets the Holy Spirit sealing him. This is a seal that cannot be broken.
Philippians 1:6
And I am sure of this, that he who began a good work in you will bring it to completion at the day of Jesus Christ. You asked "Do you sin"? And of course the answer is yes. All people sin, both saved and unsaved. I thank God for 1 John 1:8-10
If we say we have no sin, we deceive ourselves, and the truth is not in us. 9 If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. 10 If we say we have not sinned, we make him a liar, and his word is not in us.Of course there is an everyday aspect to Christianity for the rest of our lives. It isn't a say a prayer and then live the rest of your lives like a heathen. Works are vitally important; they just don't save a person.
Philippians 2:12 Therefore, my beloved, as you have always obeyed, so now, not only as in my presence but much more in my absence, work out your own salvation with fear and trembling,Matthew 24:12-13 And because lawlessness will be increased, the love of many will grow cold. 13 But the one who endures to the end will be saved.James 2:14-17 What good is it, my brothers, if someone says he has faith but does not have works? Can that faith save him? 15 If a brother or sister is poorly clothed and lacking in daily food, 16 and one of you says to them, "Go in peace, be warmed and filled," without giving them the things needed for the body, what good is that? 17 So also faith by itself, if it does not have works, is dead.In closing, the Chrisitan faith is not an easy believism. You can't just say a prayer and walk away living like the Devil for the rest of your life. Paul states in 1 Corinthians 15:31 that he "dies daily" in reference to fighting sin, and we must do the same. There is a lot more to this topic that can be put on Texags but I figured I would add this. I have enjoyed this forum because I do not engage with non-Protestants in my daily life, so this allows me to see other points of view.