We are mostly aware of the teachings of Jesus in the Gospels. We are also quite adept at living as though we never heard them. Here is a very important and oft-neglected divine teaching: "If you are offering your gift at the altar, and there remember that your brother has something against you, leave your gift there... first be reconciled to your brother, and then come and offer your gift" (Matthew 5:23-24).
To bring any sort of offering to God is to bring a symbol of ourselves. If we do not offer ourselves along with our gift, then the gift is without value in His eyes. Remember all that God said in the Old Testament about the worthlessness of sacrifices offered without any accompanying righteousness, i.e., without a right relationship with God. To offer (Greek prosphero) ourselves to God ultimately has a Eucharistic reference: it is to be like the altar bread (Greek prosphora) offered and consecrated in the Divine Liturgy.
An essential part of the Eucharistic offering is the anamnesis, or remembrance of all that Jesus has done for our salvation, all of his mysteries which will be manifested and communicated mystically in the Holy Eucharist. So we bring the offering and remember the saving works of the Lord. Then his sacrifice, which has reconciled God with man, is made present before us.
So the Lord says that when you are bringing your offering, and remember that someone has something against you, you must reconcile before God will accept your offering. The memory of Christ's works is a necessary element of the offering of the Divine Liturgy, but a memory of wrongdoing or any sort of estrangement from another person is an impediment to our personal offering, to our uniting our self-offering to the offering of Christ to his Father. Be reconciled, Jesus says, so that you can offer--so that we can enter with a pure heart into the Offering that has reconciled the whole human race with God.
Erasmo Leiva-Merikakis writes: "You cannot present your heart to God as a gift-offering on the altar of sacrifice if that heart is turned against God's other children. The way to union with God in worship cannot lead away from your brother. It is impossible for me to be a child of God without being a brother of all those for whom Christ died. I cannot love and adore God and at the same time hate and exclude God's children from my life... Christ's Incarnation, death, and Resurrection mean that he has become inseparable from those he came to redeem" (italics in original).
Note that Jesus says above, "if you remember that your brother has something against you." He doesn't specify whether that something is justifiably held against us. We also have to reconcile with those who unjustly hold something against us. Far less should we hold something against others. It may not always be possible to reconcile with a brother or sister in person, but we must at least do so in our hearts, so that we can bring a pure self-offering to the Lord as we approach the altar, as we seek to enter into Holy Communion with Him.
Anger, resentment, hatred, grudges, have nothing in common with offering, worship, and communion. Therefore remember, reconcile, then offer. And Jesus will remember you in his Kingdom, reconcile you with his Father, and draw you into communion with his all-pure and perfect Offering.