Simul Justus et Peccator is a Latin phrase that carries with it so much symbolism. "Simul Justus" means "Simultaneous Just" and "et peccator" means "and Sinner". This phrase explains how while we are yet born again and saved by God's grace, we are still sinners, who are in need of continued grace. John writes in his epistles: "Everyone who makes a practice of sinning also practices lawlessness; sin is lawlessness." and "No one born of God makes a practice of sinning, for God's[a] seed abides in him; and he cannot keep on sinning, because he has been born of God." But then John also says "If we say we have no sin, we deceive ourselves, and the truth is not in us."
So we recognize that while we as Christians are not to make a "practice" of sinning, we do still stumble and sin. This is the proper theological balance found in the Bible. Christians still sin. The other side of the coin is that we have an advocate with the Father. Right after John says "if we say we have no sin..." he adds this "If we confess our sins, he is faithful and just to forgive us our sins and to cleanse us from all unrighteousness."
So the Christian can have confidence to know that sinning is an unfortunate part of living in this fallen world. We are not free from sin, but called to live life of Holiness where we strive to resist temptations.