In the beginning, God created the heavens and the earth. A time after that, he created man and man sinned against God. When man sinned against God, he separated himself from the holiness that is God. Death entered the world and condemned all who dwelled in it. Man, in his pride, tried every way to achieve salvation. He toiled in false idols and works, yet not could save him for all were perceived as filthy menstrual rags in the eyes of a just and holy God. Through his actions, man condemned himself to the eternal fires of Hell. God, being just and holy and having no desire to see any of the man who He created burn with the devil and his angels offered a ways of Salvation. He needed a sacrifice.
So God in his wisdom sacrificed a man to for the sins of all. This man would be a perfectly innocent man whose blood would atone for the sins of everyone who has ever lived and ever will lived. But where in the world did such a man exist? Nowhere. So God came down as a man, birthed from a virgin. Having been born from a virgin, He did not know sin. Not knowing sin, sin did not exist within him. Being God and of God, He could not sin. Therefore His sacrifice was perfect. His perfect sacrifice, His perfect atonement. Praise be to Christ who was born for my sin, for your sin.