1 John 1:4 And these things write we unto you, that your joy may be full. 5 This then is the message which we have heard of him, and declare unto you, that God is light, and in him is no darkness at all. 6 If we say that we have fellowship with him, and walk in darkness, we lie, and do not the truth: 7 But if we walk in the light, as he is in the light, we have fellowship one with another, and the blood of Jesus Christ his Son cleanseth us from all sin. 8 If we say that 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 that we have not sinned, we make him a liar, and his word is not in us.
The danger of saying a christian can never sin usually winds up with false witnesses of believers since in order for them to claim that false testimony is by making sin no longer sin like they would say in 1 John 1:10 which is what John was reproving of by saying how Jesus walked should be the same as how we walk.
Since Jesus taught us a prayer to ask the Father to forgive us as we forgive others and that we are to forgive no matter how many times a trespassing believer repents and forgives: shows that even though we are called not to sin, believers that do sin are free to go before that throne of grace to ask for forgiveness and help to repent and to not sin again as that is also included in that prayer to be led not into temptation but deliver us from the evil one.... as thine is the kingdom and the power and the glory forever....
However, since God can see the thoughts and the intents of the heart better than anyone, He will judge the believer if he is willfully sinning under His grace as God is not mocked, and so those that sow to the flesh will reap corruption, which is why those that turn to Him for help will be strengthened to suffer the temptation ( Hebrews 2:18), may even be provided with an escape to bear it ( 1 Corinthians 10:12-13), be led not to make any provisions for the flesh ( Romans 13:14), be led to not be in bad company that have influence over him to tempt him (1 Corinthains 5:9-10 & 1 Peter 4:1-7), be led to cast down every thought and imagination in obedience ( 2 Corinthians 10:4-6), and be led to keep his mind on good things (Philippians 4:6-9) as He is our Good Shepherd to help us to follow Him so we will not live in sin ( Hebrews 12:1-2 & Philippians 1:6,9-11).
A believer in Jesus Christ should not practise sin nor live in sin as they will reap corruption as God is not mocked. They are still His, but like the prodigal son that gave up his inheritance for wild living, that first inheritance is gone, even though he is still son. Like Esau, he will weep: but moreso because there will be weeping and gnashing of teeth so that those left behind at the pre tribulational rapture event from participating in the Marriage Supper, will instead come out of the great tribulation needing a miracle from God to get past that great loss, and He will, by wiping the tears from their eyes Himself.
Noting how Esau had given up his birthright for something temporary as a meal, every believer should not think that not following Jesus will not have any consequences since He went through so much trouble reconciling our relationship with God by paying for the wages of our sin which was by His death.
Believers that are having trouble overcoming sin should not rationalize it nor ignore it, but turn to Him for the help they need since He is with us always as dwelling within us. We are not alone in walking through this valley of death, because Jesus Christ is our Good Shepherd as well as our Saviour.
Psalm 23:1The Lord is my shepherd; I shall not want. 2 He maketh me to lie down in green pastures: he leadeth me beside the still waters. 3 He restoreth my soul: he leadeth me in the paths of righteousness for his name's sake. 4 Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me. 5 Thou preparest a table before me in the presence of mine enemies: thou anointest my head with oil; my cup runneth over. 6 Surely goodness and mercy shall follow me all the days of my life: and I will dwell in the house of the Lord for ever.
Hebrews 4: 12 For the word of God is quick, and powerful, and sharper than any twoedged sword, piercing even to the dividing asunder of soul and spirit, and of the joints and marrow, and is a discerner of the thoughts and intents of the heart. 13 Neither is there any creature that is not manifest in his sight: but all things are naked and opened unto the eyes of him with whom we have to do. 14 Seeing then that we have a great high priest, that is passed into the heavens, Jesus the Son of God, let us hold fast our profession. 15 For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin. 16 Let us therefore come boldly unto the throne of grace, that we may obtain mercy, and find grace to help in time of need.