Atheist, your own questions are against your view, therefore, you will not have reason to say that you knew nothing about sin!
Adultery is a sin against God, in no case is divorce divorced, but the person betrayed if he wishes to send away his wife, or his husband, has the right to do so, but they do not have to marry in either, or cohabit with no one, only in this case is divorce from God!
Let's see the word of God at the bottom:
Matthew 5: 27. "You have heard that it was said," Do not commit adultery. " 28 But I tell you that anyone who looks at a woman lustfully has already committed adultery with her in his heart. 29 So if your right eye causes you to fall into sin, take it and throw it away from you; for it is better for you that one of your members perish rather than go into your whole body in hell. 30 And if your right hand causes you to fall into sin, cut it off and throw it away from you; for it is better for you that one of your members perish rather than go into your whole body in hell.
31 It was said: "Whoever repudiates his wife shall give her the act of repudiation". 32 But I tell you, whoever sends his wife away, except for the sake of fornication, makes her become an adulteress, and whoever marries her who is sent away commits adultery.
Therefore, let not man separate what God has united ".