Framing the question
I am providing an answer that I think all denominations agree on. Furthermore, I'm answering from the point of view of a disciple, defined as those who already declare Jesus to be their Lord and Savior. Within this limited context, we do not have to go into the dispute
because by definition a disciple is one who already receives Jesus's forgiveness from the cross by faith when he/she repented and converted to become son/daughter of God, and I want to provide an answer that is acceptable to both the "once saved always saved" group as well as the "we can lose our salvation" group.
So the question becomes: if we are already forgiven when we became a disciple, why does Jesus seem to condition our own forgiveness on our forgiving others?
Contextualizing the Lord's prayer and the forgiving command
In Matthew, the Lord's prayer is presented as a simple and trusting prayer model (as opposed to wordy / pretentious / vacuous prayers practiced by non-believers, cf Matt 6:5-8) containing essential daily requests that affirm our dependence on God in several areas:
- as children of God ("Our Father")
- citizen in God's kingdom ("Thy kingdom come")
- obedient to His will ("Thy will be done")
- a creature ("our daily bread")
- needing continual forgiveness from sins committed since we became Christians ("forgive us our sins")
- needing protection from evil ("deliver us from evil")
The bigger context is the Sermon on the Mount (Matt 5-7) where Jesus was emphatic that forgiving others (even enemies) IS the primary characteristic of the children of God (cf Matt 5:45), mentioning it in multiple places / contexts (even in the parables), not just in the Lord's prayer. For example:
- Reconcile before we worship God, Matt 5:23-24
- Do not retaliate even if they are wrong, Matt 5:38-42 following Jesus's example of forgiving enemies from the cross
- Love our enemies, Matt 5:43-48
We can see how Jesus's command in the Lord's prayer as the climax of his teaching on forgiveness, which he even repeats immediately after the Lord's prayer in vv. 14-15:
If you forgive those who sin against you, your heavenly Father will forgive you. But if you refuse to forgive others, your Father will not forgive your sins.
An acceptable answer by all denominations will be something like the following.
Disciples need to work out their salvation with the help of the Holy Spirit already given to them to become more and more like Christ. Therefore, we need to forgive others like Christ did his enemies, to do to others as Jesus has done to us. If our faith is genuine, we will do this, but in the weakness of our nature we need a constant reminder (thus included in the daily Lord's prayer).
Jesus's one time forgiveness on the cross needs to be realized in our consciousness and the way to do this is to realize how we are forgiven during the specific occasions when we forgive others by being Jesus to them because our sins to God are of various types and we need to realize how we have offended God and grieved others in multiple ways. By forgiving the various sins of others done against us we will then internalize instantiations of Jesus's forgiveness to us. For example, when we forgive other's unjustified anger to us, we are reminded that we need to be forgiven of our own anger. When we forgive other's betrayal, we are reminded how we have disappointed others too and need to be forgiven.
Another way to see this is that Jesus's death on the cross is a one time event of forgiveness (already received at conversion) waiting to be applied to a disciple's past, present, and future sins. Even a Calvinist who believes in TULIP cannot be certain if he/she is one of the elect. One way to have assurance is to observe his/her heart and see other's sin as playing itself out in him/herself saying:
"Looking at the root of other's sin, there may lurk in my heart similar propensity to sin that I either have done in the past or can potentially do in the future. As I forgive this person's sin against me today, I'm also applying Jesus's one time forgiveness to my particular sin as well and pray to God who is outside time to forgive me and heal me of this propensity".
If we do not do this, our faith may not be genuine, and let these refusal-to-forgive occasions come as a REALITY CHECK of our true relationship to God as children of God (a child of God would be ready to forgive). Thus, Jesus's teaching in Matt 6:14-15 can be seen as a gracious warning to help us make sure we don't deceive ourselves.