When it comes to building the temple of the Holy Spirit that we, as Catholics, are supposed to be, the first stage in construction is demolition — we must rip out anything that stands in the way of Christ.
God is perfect, and He loves us perfectly. But He asks us to love perfectly in return and, in fact, only by loving God perfectly do we allow Him to love us perfectly. Perfection is not some lofty goal, but what Christ demands.
When we ask God to create in us pure hearts, He will not necessarily do this in a gentle or pleasant manner. He will always be loving, but we should be careful what we wish for — because we just might get it!
Possessing truth in the heart is the goal of the Catholic life: to live in the truth of Christ, which is expressed in the theological virtues of faith, hope and charity.
In order to understand fully the depth of God's mercy, we must also understand His justice; and to understand that we must face and accept the reality of Original Sin.