Summary: A review of the events described int he Bible that would have to take place if a person willingly chose to lose their salvation.
Jesus said that He came to fulfill all the 613 laws and the words of the Prophets of the Old Testament and that they hang on just one commandment like a door on its hinge - "You shall love the Lord your God with all your heart and with all your soul and with all your mind" (Matt 5:17, 22:37-38; Mk 12:30; Lk 10:27 ESV). He then said that the second royal commandment was exactly the same as the first, "love your neighbor as yourself" (vs. 22:39; James 2:8). When a Born-Again Christian bears another's burdens they fulfill the Great Commandment - the law of Christ (Gal 6:2).
If a person willingly chooses to lose their salvation, after receiving Jesus Christ as their personal Lord and Savior and repenting of their sin (Acts 2:38, 16:31; Rom 10:9-10), the common-sense, logical reality is that God, the Father, would have to ask Jesus, God the Son, to give back His gift of love to Him and then go back on His promise to never cast them out (Jn 6:37) and cut them out of His Will as co-heirs with Jesus (Rom 8:17). The person would have to un-drink the water of eternal life given by Jesus and so that they will be thirsty again (Jn 4:14).
Jesus would have to quit as their defense attorney (1 Jn 2:1); no longer protect or keep them safe (Jn 17:9-12); divorce them (Rev 19:7); un-adopt them (Rom 8:23); rescind His promise to make them a citizen of Heaven (Jn 15:16; Eph 2:12-13,19) because they are now an un-new creation (2 Cor 5:17); make them un-near to Him (Eph 2:12-13,19); un-crucify their old nature (Gal 2:20); blot out their names from the book of life (Rev 3:5); un-bury them and un-baptize them so that they will no longer walk in newness of life (Rom 6:4); put them back into the world (Jn 17:16); un-circumcise them (Col 2:11); and un-complete them (Col 2:9-10).
Jesus would also have to allow death and life, angels and rulers, things present and things to come, powers, height, and depth, and anything else in all creation, to separate them from the love of God found in Him (Rom 8:38-39) and no longer be their advocate before the Father (1 John 2:1); un-buy and refund the payment He made by His blood; un-predestine them so they can no longer be conformed to His image; un-sanctify and un-deliver them from the domain of darkness and un-transfer them out of His kingdom, where they were once conditionally sanctified daily (Heb 10:14; Col 1:13; Rom 8:28-31; Eph 1:11); un-call, un-justify and un-glorify them (Rom 8:25, 28-31); un-cancel all their spiritual debts (Col 2:13-14); un-seal them (2 Cor 1:22, 5:5; Eph 1:13, 4:30), un-wash and un-forgive them of all their sins (Titus 3:5; 1 Cor 6:11); re-remember their sins (Heb 10:17); un-consider them holy and blameless in His sight (Eph 1:4, 2:8; Rom 8:9,11:6); un-buy and un-born them of God who then moves out from living within them as the new Holy of Holies (1 Cor: 6:19-20, 7:23-24; 1 John 2:19, 3:6,9; Rom 8:9); rescind and fail to honor His promise to never leave and forsake them (Heb 13:5); revoke their position of no longer living in His new nature (Rom 8:9) go back on and fail to honor His promise to never cast them out (Jn 6:37).
God, the Holy Spirit, who was given as a guarantee (2 Cor 5:5) would have to un-save them, un-fill them (becoming un-Born-Again) (Col 2:9-10); un-anoint them (1 John 2:27) and stop His good work begun in them and fail to honor His promise to keep them from stumbling until the end when Jesus comes again (1 Cor 1:8; Phil 1:6, 2:13; Jude 1:24-25).
To say that an individual could be separated from the love of God or remove themselves from His firm grasp, and from His promise to NEVER leave them after He bought and paid for them, and they are His possession, as well as His dwelling place, the Holy of Holies, is at the very least, illogical and intellectually incongruous. When God makes a promise He NEVER reneges on, or breaks, His word.
For those who were never Born-Again, yet clearly understood the message of Jesus, and had a taste of the heavenly gift of God's Word and walk away, it is IMPOSSIBLE to restore them again to repentance, since they are crucifying once again the Son of God to their own harm and holding him up to contempt (Heb 6:4-6 ESV). Yet, the Bible says that when the Born-Again Christian sins (and they will conditionally because they are just saved sinners) they "have an advocate with the Father, Jesus Christ the righteous" and He will not cast them out (1 John 2:1).
In the end, even though it would appear the Bible is very clear that those who practice sin (bad fruits) and leave the faith, were never with Jesus, and He never knew them, because they would have continued with Him (Matt 7:16, 13:1-9, 18-23; Rom 8:13; 1 Jn 2:19, 3:7-10), the free will God gave all human beings is in fact, the freedom of willful choice, it would appear logical to say, even though salvation is impossible with man (Mk 10:27), that the possibility for a Born-Again Christian to eternally reject their salvation, exists. How, or when, that line is crossed, can be known only by God.