"Keep thy heart with all diligence; for out of it are the issues of life." Proverbs 4:23
The word issues is also translated fountain. Our heart has a "fountain of life" in it. That fountain is Jesus-- the Way, the Truth and the Life. His life is suppose to be issuing (flowing) from our hearts so that others can taste and see that He is good. It should be flowing so that they will know that He quenches the thirst of our souls.
The Bible lets us know that our hearts have a "door" on them. Sin is always crouching at the door awaiting access. (Genesis 4:7) When we open the door to sin, sin will come in. Once it does, it starts taking over our hearts. It starts holding the life that is in us captive so that it can't freely flow from us.
For example, when bitterness comes in, it takes love captive. We are so taken over by bitterness that love can't flow from our hearts. When fear is allowed in, it takes faith captive-- faith can't flow from us. When we open the door to confusion, peace can no longer flow from our lives. That is the way it is with any of the characteristics of sin. It demands to rule the whole heart and takes captive anything that is there.
This is why we must guard our hearts from opening the door to anything that is connected, or related, to sin. We must be diligent and watchful because sin is crouching, that means it is in the "ready, set, go" position. It will move so fast to gain access that it is in the door before you know it. Keep on guard. Keep watchful. Keep attentive. Don't let down your guard. Keep your heart full of the life of Jesus. Keep your heart full of the Spirit. When you do, the door will stay shut to keep out sin. And when it is open to release the fountain of life in you, that fountain will be so strong that it will be like a "river" that will carry sin away from the door and down stream.
Have a great day. Guard your heart. Don't let sin in to hold it captive.