Bede the Venerable — as excerpted in the Catena Aurea on Mark 11:19-26

“(ubi sup.) But we must observe that there is a difference in those who pray; he who has perfect faith, which worketh by love, can by his prayer or even his command remove spiritual mountains, as Paul did with Elymas the sorcerer. But let those who are unable to mount up to such a height1 of perfection pray that their sins should be forgiven them, and they shall obtain what they pray for, provided that they themselves first forgive those who have sinned against them. If however they disdain to do this, not only shall they be unable to perform miracles by their prayers, but they shall not even be able to obtain pardon for their sins, which is implied in what follows; But if ye do not forgive, neither will your Father which is in heaven forgive your trespasses.”