The human heart has so many crannies where vanity hides, so many holes where falsehood lurks, is so decked out with deceiving hypocrisy, that it often dupes itself. John Calvin

The Puritan pastor John Calvin identified that our hearts are places where pride lurks, where lies prevail to the point we deceive ourselves in not recognizing our own heart problems. The pride in our hearts shows itself when we decide that what we want is more important than glorifying God. The Bible uses words like lusts, passions, desires, longings, and pleasures to describe what our heart is set on.
These are the things that lead us into dangerous waters, and lead us to sin in thought and deed. 
But each one is tempted when he is drawn away by his own desires and enticed. Then, when desire has conceived, it gives birth to sin; and sin, when it is full-grown, brings forth death. James 1:14-15 (NKJV)

We usually do not give our thoughts, beliefs and desires much thought as being sinful because as Calvin said we tend to deceive ourselves as to our heart’s true condition. In truth, we can rationalize and justify anything!

James 1:13-14 explains to us how we sin-how the desires of our sinful hearts, give way to sin. The Greek word for tempted is described as being a hook, or a lure for a fish. So our own desires act like bait on a hook that lead us to sin in the ways we want to. It is rare we tell ourselves, “I am going to sin now and I don’t care.” Usually, we conceive a thought and give it a passing glance past our conscience or the Holy Spirit on the way to action.

Occasionally we catch it (depending on how spiritually mature we are, or on how determined we are to obey God) and stop ourselves even at that point. Unfortunately for us our sinful habits are so ingrained that we often react without much conscious thought.

This is not to say that we are not responsible for our actions, or that some entity outside ourselves is “making” us sin, we are always responsible and accountable for our actions and thoughts. That is why it is so critical to understand the affections of the heart, and how these thoughts, beliefs, and desires must be changed from those that are focused on our selfish, sinful selves to those that are focused on loving and glorifying God and serving others.
Look at what Paul writes about the desires of the heart and the outcome of following them-
When you follow the desires of your sinful nature, your lives will produce these evil results: sexual immorality, impure thoughts, eagerness for lustful pleasure, idolatry, participation in demonic activities, hostility, quarreling, jealousy, outbursts of anger, selfish ambition, divisions, the feeling that everyone is wrong except those in your own little group, envy, drunkenness, wild parties, and other kinds of sin. Let me tell you again, as I have before, that anyone living that sort of life will not inherit the Kingdom of God. Galatians 5:19-21 (NLT)
It is important to say here, that verse 21 says that those who live this way in a habitual way won’t inherit the kingdom of God because only an unbeliever could follow these desires as a lifestyle! A true believer in Jesus Christ who has been redeemed by His blood will not make this a way of life and be unaffected by it. The Spirit of God that indwells the believer would be making the person miserable beyond words and repentance would take place. We must keep this in mind when we see people who have “prayed the prayer” or “made a decision for Christ” but have absolutely no evidence (fruit) of the Spirit’s indwelling presence in their lives!
I maintain that rather than trying to convince them and ourselves that they will get better, we had really help them to see that in spite of their prayer or decision they may not be truly redeemed at all! To do otherwise is cruel!
Paul is warning the Galatians that those who practice these things are in danger of the souls being lost because he is questioning their conversion in the first place. He is saying that practice of these things are inconsistent with a heart changed by the Holy Spirit. Yes, Christians still sin, but we ought take it seriously and be working to put off the desires of the sinful nature that still come from hearts.
There is only one cure for what ails us within, that is falling upon the grace and mercy of the Lord Jesus Christ.