13 Thus, also, when He gave the law of prayer, He added, saying, “And when you stand praying, forgive, if you have ought against any; that your Father also which is in heaven may forgive you your trespasses.” And He calls back from the altar one who comes to the sacrifice in strife, and bids him first agree with his brother, and then return with peace and offer his gift to God: for God had not respect unto Cain's offerings; for he could not have God at peace with him, who through envious discord had not peace with his brother. What peace, then, do the enemies of the brethren promise to themselves? What sacrifices do those who are rivals of the priests think that they celebrate? Do they deem that they have Christ with them when they are collected together, who are gathered together outside the Church of Christ?
Source: The Treatises of Cyprian (New Advent)