7 What has already been said in favour of charity holds good for the inculcation of the pardoning of injuries which is no less solemnly commanded by the Lord: "But I say to you, love your enemies; do good to them that hate you; pray for those that persecute you and calumniate you, that you may be the children of your Father who is in Heaven, Who maketh His sun to rise upon the good and the bad". Hence that terribly severe warning of the Apostle St. John. "Whosoever hateth his brother is a murderer. And you know that no murderer hath eternal life abiding in himself."
Source: Pacem, Dei Munus Pulcherrimum (Vatican.va)