Psalm 51:10 says, "Create in me a pure heart, O God, and renew and steadfast spirit within me."
Take notice of some things in this very humble and important prayer of David's. First, David requested of the Lord two things, "a pure heart" and "steadfast spirit". The first appeal David did not have at all so he asked God to "create" in him this characteristic of "a pure heart". The second attribute, a "steadfast spirit", David had at one time for he asked God to "renew" it "within" him. The point is David was willing to humble himself and request of God that which he was in deficit on both accounts.
Humanity was dealt a self-replicating code, planted illegally, on purpose, to damage and/or possibly destroy humans altogether. The culprit was the Father of Lies, Satan. The code injected into humanity was a virus of disobedience to God or Sin.
Therefore, the instruction from God in Scripture to remain pure after accepting His gift of salvation should come as no surprise. Additional Scripture advise believers to keep their minds pure (Philippians 4:8) and know the pure in heart will be rewarded by seeing God (Matthew 5:8).
In particular, God points to three areas of the body that need constant purity monitoring and application. These corrupted areas are the heart (generator), the eyes (input and output) and the tongue (output) respectively.
The heart is an organ that feels the effects and is considered the prime descriptive word for the source of our emotional, passionate, ambitious and zealous responses. So many times we allow the judgments of our hearts/emotions to outweigh our mind or simply not come into balance with our logic. In Matthew 15:18-20, Jesus reveals just how bad the condition of the human heart or emotional system is when He says,"But the things that come out of a person’s mouth come from the heart, and these defile them. For out of the heart come evil thoughts—murder, adultery, sexual immorality, theft, false testimony, slander. These are what defile a person; but eating with unwashed hands does not defile them."
- corrupt the whole body
- ruin your future (set the whole course of one's life on fire)
- be used by the kingdom of darkness (is itself set on fire by hell)