What does it mean to fall from the grace? I've doubted Jesus so many times, I'm scared I've fallen. I pray to God to forgive me and ask Him to never let me go, never let me take my eyes off Christ or I'll be lost. How do I know I haven't fallen?
"Indeed I, Paul, say to you that if you become circumcised, Christ will profit you nothing. And I testify again to every man who becomes circumcised that he is a debtor to keep the whole law. You have become estranged from Christ, you who attempt to be justified by law; you have fallen from grace" (Galatians 5:2-4).
In the context which Paul used the phrase, he was telling the Galatians that when they tried to bind parts of the Old Law on Christians, they were separating themselves from Christ and had fallen from the grace that he delivered to men. Salvation comes because Christ died for us. Access to that salvation was under the terms of grace in the New Law. So to abandon the New Law for the Old Law would mean a person had left grace behind.
"But also for this very reason, giving all diligence, add to your faith virtue, to virtue knowledge, to knowledge self-control, to self-control perseverance, to perseverance godliness, to godliness brotherly kindness, and to brotherly kindness love. For if these things are yours and abound, you will be neither barren nor unfruitful in the knowledge of our Lord Jesus Christ. For he who lacks these things is shortsighted, even to blindness, and has forgotten that he was cleansed from his old sins. Therefore, brethren, be even more diligent to make your call and election sure, for if you do these things you will never stumble; for so an entrance will be supplied to you abundantly into the everlasting kingdom of our Lord and Savior Jesus Christ" (II Peter 1:5-11).
Being a Christian is not about never sinning, but about constantly growing. In that growth we become more resistent to sin. Sins happen at times, but God gives His children a way to deal with sin. "If we say that we have no sin, we deceive ourselves, and the truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His word is not in us. My little children, these things I write to you, so that you may not sin. And if anyone sins, we have an Advocate with the Father, Jesus Christ the righteous" (I John 1:8 - 2:1).
How do you know you are saved, the book of I John addresses the issue, but consider this passage:
"Now by this we know that we know Him, if we keep His commandments. He who says, "I know Him," and does not keep His commandments, is a liar, and the truth is not in him. But whoever keeps His word, truly the love of God is perfected in him. By this we know that we are in Him. He who says he abides in Him ought himself also to walk just as He walked" (I John 2:3-6).
God hasn't hidden anything from us. He has told us what we need to do. So we learn His word, apply it to our lives, and grow closer to Him.