1 John 5:13 "I have written these things to you who believe in the name of the Son of God, so that you may know that you have eternal life."
How can I know I have it if I can lose it and how can I lose it if it's eternal. The apostle Paul also states that "what God has begun, He will finish." God began the work of salvation in me and He will complete it. Receiving salvation is not dependent on my works; neither can keeping it be by my works.
The new man created by God in righeousness and holiness at the new birth cannot sin, it is of God. The old man who is corrupt is the one who sins. The new man will eventually overcome the old man in Christ. "Greater is He that is within me, than he that is in the world." It is God who saved us, and it's God who keeps us.
I once believed one could lose his salvation until through study of God's Word and the Holy Spirit set me free from such fear. "God has not given us a spirit of fear, but of love and a sound mind." I am completely secure in Christ.
1Jn. 5:13 does indeed promise assurance, but It is one thing to believe that one is saved by faith, and another thing to believe that faith in the promise is salvific even if one lives an unrepentant life of sin. The “these things” of 1Jn. 5:23 refers to what John wrote previous to it, which is a description of what manner of faith denotes a true believer. As he wrote in his gospel, “ My sheep hear my voice, and I know them, and they follow me: And I give unto them eternal life; and they shall never perish, neither shall any man pluck them out of my hand.” (Jn. 10:27,28) This is a promise given to believers, “who are kept by the power of God thru faith unto salvation.” (1Pt. 1:5) Faith is the key.
As for losing salvation, while it is not by works, it is appropriated by an act of God-given faith, and thus recanting that faith, as the Galatians were warned of doing, (Gal. 5:1-4) would signify a forfeiting of what faith appropriated.
Thus a one time faith decision does not provide assurance to those who deny the faith, which believers can do. (1Tim. 5:8) It is therefore that God chastens disobedient saints, so “that we should not be condemned with the world.” (1Cor. 11:32) If He did not, or if we do not respond in repentance, then we would be condemned.
Thus OSAS must both reject the increasing but old heresy that one can be saved and live anyway his flesh pleases, as long as he believes the promise of eternal life to believers, and must also believe that while the elect can fall into a life of faith-denying sin, God will always successfully work repentance. And that while such passages as Gal. 5:1-4 and Heb. 10:18-39 may warn of believers denying the faith and that which is procured, it is a reality that would never occur.
As i seek to honestly look at Scripture, I see the whole of it allowing that believers could do so, but that we must trust God will chasten us back to saving faith, if we so depart, though Heb. 6:1-10 may disallow that in the case of willful departure from the faith.