Hosea

For me personally, the book of Hosea is the most convicting in all of scripture. When you compare the relationship between humans and God to the relationship between a man and a woman, we can really see the weight of our sins. It also helps me grasp the concept that all sins are equal in God's eyes. When a spouse betrays their significant other, the specific act that takes place means very little. It's the intention behind it. It could be a "big" act or a "small" act, but in the heart of the betrayed, all they feel is that their beloved has given their attention, focus and affection towards somebody else when it was meant to be directed at them.

When God sees our sin, I think He feels the same way. It's not necessarily the action that bothers Him, but the condition of our heart. Whether we tell a "simple lie" or commit a murder, in God's eyes, we're giving our affection and focus to sin instead of directing it at Him.

It also helps us understand why we shouldn't sin. It's not to win brownie points with God or to impress our Christian friends. It's because our sin breaks the heart of God. If we claim to love God, why would we want to put Him in the situation where He feels "she decked herself with rings and jewelry, and went after her lovers, but me she forgot." (Hosea 2:13)

But God, in His mercy, is eager to forgive us and take us back. 

Below in a great retelling of the story of Hosea by Reggie Dabbs, an incredible youth speaker: