A pure heart is the kind of heart that God wants from us; a heart free of strive and grudges. If we can ourselves christians and we still hold unto one one two people in our hearts then we are not getting it right because this should not be how a believer should act.
1 John 3:15
Whosoever hateth his brother is a murderer: and ye know that no murderer hath eternal life abiding in him.
The "brother" in the above passage is not just referring to our biological sibling. It goes out to our brethren in faith and everyone generally in life. Instead of harbouring hate or grudges in our heart we should replace it with love. When we have love in our hearts we can forgive people no matter what they may do to us.
God does not want us to walk in unforgiveness. It is the opposite of what He wants from us. Forgiveness is not for the good of the people we are forgiving. It is actually for our own good. It sets of free from every heaviness that the heart should go through and makes us aright with God.
