Why did Jesus say He won’t forgive us if we don’t forgive?

Regarding Matthew 6, Jesus is speaking temporally, not eternally. Notice the context:

Matt. 6:14 “For if you forgive others for their transgressions, your heavenly Father will also forgive you.
Matt. 6:15 “But if you do not forgive others, then your Father will not forgive your transgressions.

Jesus said if we forgive someone, then the Lord will forgive us. Jesus equates the two types of forgiveness. God's forgiveness of us is contingent on our forgiving others. As we try to understand Jesus' words, we must first remember that we have no power over the eternal fate of another person, for Christ alone is our Judge:

Acts 10:42 “And He ordered us to preach to the people, and solemnly to testify that this is the One who has been appointed by God as Judge of the living and the dead.

We know Jesus wasn't asking us to forgive someone from the penalty of their sin in eternity for only God can do that. The only kind of forgiveness we can extend to another person is the restoration of our earthly fellowship with that person. So when Jesus asks us to forgive another, He must be speaking in terms of earthly relationships, not eternal outcomes.

If a believer forgives in this way, Jesus says God will forgive us. From the context, the proper interpretation is that God will forgive us in the same way He asks us to forgive others: by restoring fellowship with us. Once again, Jesus is speaking in terms of earth consequences and earthly forgiveness. The Lord will forgive us from the earthly consequences of our sins provided we are willing to show forgiveness to others (which is a part of our mission to show the love of Christ to the world).

The Bible teaches that our sins after faith may still bring us earthly consequences. The Lord may choose to bring discipline against us for our sin, which the Bible calls the discipline of the Lord. Discipline is intended by God to drive us back into a godly walk with Him:

Heb. 12:7 It is for discipline that you endure; God deals with you as with sons; for what son is there whom his father does not discipline?
Heb. 12:8 But if you are without discipline, of which all have become partakers, then you are illegitimate children and not sons.
Heb. 12:9 Furthermore, we had earthly fathers to discipline us, and we respected them; shall we not much rather be subject to the Father of spirits, and live?
Heb. 12:10 For they disciplined us for a short time as seemed best to them, but He disciplines us for our good, so that we may share His holiness.
Heb. 12:11 All discipline for the moment seems not to be joyful, but sorrowful; yet to those who have been trained by it, afterwards it yields the peaceful fruit of righteousness.

The means of obtaining earthly relief from the Lord's discipline is confessing our sins and seeking the Lord’s forgiveness, which the Lord will grant us as John promises:

1John 1:9 If we confess our sins, He is faithful and righteous to forgive us our sins and to cleanse us from all unrighteousness.

When we repent and confess our sins (as a believer), the Lord will grant us relief from His discipline, but Jesus says in Matthew 6 that if a believer is so hardhearted that he or she withholds forgiveness in earthly relationships, then the Lord will withhold His forgiveness for that person's sins as well, which means the Lord will visit earthly discipline against such a hardhearted believer.

The believer's eternal salvation is not impacted, for Jesus isn’t speaking of the eternal forgiveness that comes by faith alone, nor is Jesus giving the Church a recipe for maintaining our salvation, for no believer can sin his or her way out of salvation. God’s grace is greater than our sin:

Rom. 5:20 The Law came in so that the transgression would increase; but where sin increased, grace abounded all the more,
Rom. 5:21 so that, as sin reigned in death, even so grace would reign through righteousness to eternal life through Jesus Christ our Lord.

The believer is forever forgiven the eternal consequences of sin (i.e., the Second Death) from the moment of his or her faith in Jesus Christ, and this can never change.

Therefore, Jesus is speaking about earthly relationships and earthly consequences. If we desire the Lord to forgive us of our daily missteps and to withhold His discipline, we must be prepared to extend our forgiveness to others for their missteps against us.

Scripture quotations taken from the (NASB®) New American Standard Bible®, Copyright © 1995, 2020 by The Lockman Foundation. Used by permission. All rights reserved. www.lockman.org