We can live a life of purity with God's grace, we only need to ask and accept it. Yet when we think we can do things our way and not God's way, we reject His grace. Without God's grace, we cannot be pure of heart and we will sin. There is no other way, but to be in a state of grace if we want to be with God. Purity through grace gets us there.
"If I be not in a sate of grace, I pray God place me in it; if I be in it, I pray God keep me so." - St. Joan of Arc
"Above all, the state of grace is absolutely necessary at the moment of death; without it, salvation and supernatural happiness the beatific vision of God - are impossible." Pope Pius XII
"A soul in a state of grace has nothing to fear of demons who are cowards." - St. Theresa of Lisieux
"For grace is given not because we have done good works, but in order that we may be able to do them." - St. Augustine
"The key is to persevere in a state of grace. If one is able to stand firm in the midst of temptation and keep pure one's heart, the Holy Spirit will teach such a soul the mysteries of God. But the first step is conversion... to turn from mortal and then venial sin." - Fr. Ian VanHeusen
What is Sanctifying Grace? Sanctifying grace is that grace which makes the soul holy and pleasing to God." - Baltimore Catechism