Purity.
It's a quality that we desire when something when we are looking at something of value. If a piece of jewelry says "pure gold," and we paid serious money for it, and come to find out it's mixed with lead (which mimics the weight of gold, because it's almost as dense) we will feel deceived.
It's a quality that we desire if we are consuming something. If the bread says, "100% whole wheat" and we find it has white flour in it, we feel that we paid for something we did not get. If it says, "gluten free" and has wheat flour in it, I personally will get sick and if someone has celiac disease, they could end up in the hospital.
We want the label, "pure," to be accurate as consumers. Governments passed all sorts of laws in the last few centuries to ensure that labels matched the contents.
But if you look up the word, "pure," in Strong's Concordance, you find it applied to the furniture and religious items that God commanded to be made by His hand-picked craftsmen in Exodus 31. Then you see an immediate counterfeit of that in Exodus 32, where gold is used to craft an idol: the golden calf.
The world always counterfeits the things of God. Satan counterfeited true wisdom by insinuating to Eve that God was holding out on her, so she needed to grab the apple and get "real" wisdom. Satan will take the gold of God--His love, wisdom, provision, and instruction--and add impurities to it (sin, pride, arrogance, deception) and try to pass it off to us as real.
We may not discover how impure Satan's counterfeits are until the consequences come at us--for Adam and Eve it was expulsion from the Garden into a world where the first act outside of it was the murder of one son by the other son.
Once again, Jesus is distilling the Kingdom of God's charter with what we call the Beatitudes, or what someone once called, the "beautiful attitudes." This one is Matthew 5:8:
"Blessed are the pure in heart,for they will see God."
- Free from corrupt desire, from sin and guilt
- Free from every admixture of what is false, sincere genuine
- Blameless, innocent
- Unstained with the guilt of anything [1]
Have mercy on me, O God,
according to your unfailing love;
according to your great compassion
blot out my transgressions.
Wash away all my iniquity
and cleanse me from my sin.
For I know my transgressions,
and my sin is always before me.
Against you, you only, have I sinned
and done what is evil in your sight;
so you are right in your verdict
and justified when you judge.
sinful from the time my mother conceived me.
Yet you desired faithfulness even in the womb;
you taught me wisdom in that secret place.
Cleanse me with hyssop, and I will be clean;
wash me, and I will be whiter than snow.
Let me hear joy and gladness;
let the bones you have crushed rejoice.
Hide your face from my sins
and blot out all my iniquity.
Create in me a pure heart, O God,
and renew a steadfast spirit within me.
Do not cast me from your presence
or take your Holy Spirit from me.
Restore to me the joy of your salvation
and grant me a willing spirit, to sustain me...
My sacrifice, O God, is a broken spirit;
a broken and contrite heart
you, God, will not despise. (Psalm 51:1-12 & 17)
This psalm was written after David committed adultery with Bathsheba. So, David was not on the "Oh, I have overcome this!" side of this! But this psalm is a reminder of how God can cleanse, renew and transform a heart. Even if that heart fails time and time again.