I in no means justify sin in a believer but Jesus said
John 8:7 KJV
So when they continued asking him, he lifted up himself, and said unto them, He that is without sin among you, let him first cast a stone at her.
Everyone of us sin. If you claim you're a Christian, then you sin as well. Lying, judging, stealing, being fearful, worrying, and such things are sin. There is no one in this world that is exempted to sin. Even Job the most righteous man on earth is sinful as well. Through the nature of sin we inherited from the fall of man, we became sinful. But Jesus' sacrifice gave us a way to redemption. But even Peter, as he judged the gentiles for being uncircumcised had sinned through that act. He was a preacher, an apostle of Christ.
Romans 3:23 KJV
For all have sinned, and come short of the glory of God;
The Lord knows our weaknesses but the Lord is not expecting us to be perfect.
That's why for Christians who sin, it is always needed to repent as we sin everyday through our thoughts, actions, judgments, heart's intention.
Paul wrote this
1 Corinthians 15:31 KJV
I protest by your rejoicing which I have in Christ Jesus our Lord, I die daily.
Paul said that as even tho he was forgiven and believed the Lord Jesus Christ whom he once persecuted, he still sin so through repentance, he die from his sins