Those who sin are of the devil

Jesus Christ came to set us free from sin. Those whom the Son of God has set free will be free and free indeed. Nobody who follows Jesus keeps on sinning. This is how you see the difference between those who belong to Jesus and those who are of the devil.
A child of God will not sin because the seed of life in Christ Jesus has been planted in him and he seeks to please Jesus. The Holy Spirit that Christ gives in the believer continuously convicts of sin, righteousness and judgment; and leads us into absolute obedience to Jesus.
Those who have committed themselves to Jesus continuously respond to the conviction of the Holy Spirit. Whenever they do wrong, they repent, plead forgiveness and avoid sin at all cost. Thus they become holy as Christ is holy. The constantly guard against grieving the Holy Spirit through disobedience.
He who is a sinner at heart ignores the Holy Spirit and continues in his sin, seeking for some justification, a scripture or an excuse. They remain in sin. Many of them are religious, hypocrites, they know the scriptures but they do not obey the scriptures.
Jesus knows our hearts and He judges even our intensions. Nothing is hidden from Him. He will judge us righteously, by His words as written in the gospels, Matthew, Mark, Luke and John; and also in our hearts. Nobody will have any excuse.
Faith without works is dead. If we go on sinning and do not become holy we prove by our works that we are children of the devil.