Blessed are the pure in heart, for they shall see God. Matthew 5:8
Why does Jesus say we should be "pure in heart? The reason is because our heart-our inner being - is the root of all our actions. From our hearts come our motives, our desires, our goals, our emotions. If our hearts aren't right , neither will be our actions.
Jesus put it this way: "From within, out of the heart of men, proceed evil thoughts, adulteries, fornications, murders, thefts, covetousness, wickedness, deceit, lewdness, an evil eye, blasphemy, pride, foolishness" (Mark 7:21-22)
Not a very pretty picture!
But God wants to give us a pure heart and He will. He does this first of all when we turn to Christ in repentance and faith, for "the blood of Jesus Christ His Son cleanses us from all sin"(I John 1:7). But He does it also day by day, as we submit to the Holy Spirit and with His help flee from evil and seek what is good. "Blessed are the pure in heart."