In his Gospel, St. Matthew recollects how Jesus spoke of the necessity for a person to reconcile with his brother before offering sacrifice in the temple.  Though not a particularly "holy writer", Peter Abelard (d. 1142), French theologian and philosopher, did contribute some insightful treatises on religious subjects at a certain point in his life.  In his little work on the Our Father, he wrote strikingly of those who pray without forgiving:

" 'Forgive us our debts, as we forgive those who owe debts to us.'  How this prayer must be feared, my brothers, how ruinous it is!  For it confers more loss than gain, more injury than advantage.  There are some who, hardened by long and great hatred, or through terrible malice, wish very strongly to strike or slay with their own hands those who have made themselves their debtors through some injury.  They wish neither to receive satisfaction through the entreaties of men nor to make peace out of the fear of God.  Concerning these, it is written, in Deuteronomy 32, 32: 'Their grapes are grapes of gall, their clusters are most bitter.  Their vines are the gall of dragons and the incurable venom of asps.'  In secret those who are such come together to the church and pray before God and his altar, saying, 'Forgive us our sins.'  O miserable stupidity!  O wretched presumption, to provoke the wrath of God against themselves by their prayers!  A man nurses his wrath against a man, and he seeks mercy from God?"