So, because you are lukewarm—neither hot nor cold—I am about to spit you out of my mouth
Therefore, entrance to Heaven is barred when one has an unforgiving heart. It is a Kingdom principle and cannot be altered just as gravity on this planet cannot be altered.
The Parable of the Unforgiving Servant in Matthew conveys our relationship with the Father and with others. Jesus paid our debt that He didn't owe and that we ourselves couldn't pay.
Because of this, the Father has forgiven us this unpayable, unsurmountable debt just as the "certain king" forgave the debt of the servant who owed much.
Preceding the above parable note that Peter had asked of Jesus:
21.. "Lord, how many times shall I forgive my brother when he sins against me? Up to seven times?"
Matthew 6:12 And forgive us our debts, As we forgive our debtors.
Bearing feelings of unforgiveness never harms the source/perpetrator. It only harms the one harboring it.
In fact, the object of the unforgiving attitude is usually virtually unaware and totally unaffected while the one holding the unforgiveness in their heart is both emotionally and spiritually miserable and in danger of the fires of hell.
To clarify, harboring unforgiveness only hurts humankind while on this planet by removing our peace and for many can actually cause an emotional "hell on earth" as well as the potential of actually literally sending one to hell after death.
Jesus made it very clear that our sins are not forgiven us if we do not forgive others. If our sins are not forgiven, we cannot enter into the presence of God as the holy and righteous God cannot tolerate sin in His presence.