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.
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.
Related verse:
Revelation 3:5 (New International Version) 5He who overcomes will, like them, be dressed in white. I will never blot out his name from the book of life, but will acknowledge his name before my Father and his angels.
This means to me that ones name must first be put in the book life before it is to be "blotted out".
If they depart or recant the faith as you suggest, they were not genuine to begin with. They professed only, but in actually did not “have eternal life.” They tasted, they heard, and knew the message, and experienced the convicting power of the Spirit, yet never truly repented, accepting Jesus Christ as Lord and Savior. That which is “born of God,” the new creature in Christ, cannot and will not ever deny Him.