Faith—What is it?

Sins of the heart. What does this mean? You may have heard of this term before, but not really understand what its meaning is. I will do my best to try and explain my understanding of this term.
In the Old Testament God gave commandments they were meant for us to follow. These commandments were of an external sin. We will take two of them as an example of what I will be trying to put across.
In the Old Testament these commandments (or sins) were an external sin. Meaning that in order to break this commandment, you had to really commit murder or commit adultery. This Old Testament law will be changed after Jesus comes to the world in flesh.
In the New Testament Jesus comes to us in the form of a man and brings change. This change, when you really get to the center of it, is so big of a change that it requires a complete new way of thinking and living. The central focal point of this new change, Jesus gives us in John chapter 13 verse 34; “ A new commandment I give to you, that you love one another: just as I have loved you, you are also to love one another.”
This new commandment takes the sin from an external view to an internal view. Love is of the heart. Let me explain with some more from the word of God. Remember when I said I would use two of the commandments in example. Well here goes:
With these two examples, you can see the changes that have taken place between the old and the new. Christ has given a command that requires a complete new way to live your life. No longer is committing a sin just an external act, that is something that would be done on the outside. As Catholics, we know the two sins that I use as an example here are considered mortal sins, to kill someone or to commit adultery. But in this light where hating someone or being truly angry with someone is viewed in God's eye as the same as a murderer, or to just to think of someone in a lustful manner, is the same as actually being an adulterer. This brings a new meaning to what a mortal sin is.
Sin is moved to an action of the heart as well as external actions. We know that Jesus tells us that we need to love everyone as he loves us. This is one thing concerning love and can be found in Romans chapter 13 verse 9 and 10. “For the commandments, You shall not commit adultery, You shall not murder, You shall not steal, You shall not covet,” and any other commandment, are summed up in this word: You shall love your neighbor as yourself. Love does no wrong to a neighbor; therefore love is the fulfilling of the law.”
With this kind of thinking, there may be a lot of contemplating we may need to do when it comes to how and what we think is mortal and venial sin.