If I don’t forgive others, can I lose salvation according to Matthew 6:15?

Before we answer this question, we must first remember the words of 2Peter 1:

2Pet. 1:20 But know this first of all, that no prophecy of Scripture is a matter of one’s own interpretation,
2Pet. 1:21 for no prophecy was ever made by an act of human will, but men moved by the Holy Spirit spoke from God.

Peter reminds us that scripture always has one proper interpretation. Its meaning is not subject to our personal views or desires. In other words, we can’t make it mean whatever we want it to mean. Its meaning is determined by God Who authored the words for us. Peter says that only men “moved” by the Holy Spirit can bring forth scripture, and by the same token only the Holy Spirit can lead us to a proper interpretation of His own words. 

Secondly, all believers possess the Teacher, the Holy Spirit, living in them as Jesus promised:

John 14:25  “These things I have spoken to you while abiding with you.
John 14:26 “But the Helper, the Holy Spirit, whom the Father will send in My name, He will teach you all things, and bring to your remembrance all that I said to you.

So all believers possess the Spirit of God, which means all believers possess the potential to know all scripture. Nevertheless, believers will not have a perfect understanding of the Bible this side of Heaven, because the Lord doesn’t reveal His whole counsel to every believer. As Paul says:

1Cor. 13:12 For now we see in a mirror dimly, but then face to face; now I know in part, but then I will know fully just as I also have been fully known.

In the meantime, the Lord equips some within the church to teach others concerning the meaning of His word. The presence of teachers in the Church doesn’t replace a believer’s responsibility to study personally, but they will aid in our personal study, so we should seek them out wisely, as Paul says:

Eph. 4:11 And He gave some as apostles, and some as prophets, and some as evangelists, and some as pastors and teachers,
Eph. 4:12 for the equipping of the saints for the work of service, to the building up of the body of Christ;
Eph. 4:13 until we all attain to the unity of the faith, and of the knowledge of the Son of God, to a mature man, to the measure of the stature which belongs to the fullness of Christ.

Differences of opinion concerning scripture simply evidence that we lack a full understanding of God’s word, and therefore when we encounter such difference in opinion, we return to scripture seeking clarification from the Lord. He may not provide that clarification immediately, but as we continue to seek Him in His word, the answers will come. Eventually, we receive a full understanding in our glorified state as Paul said in 1Cor 13.

Regarding Matthew 6, Jesus is speaking temporally, not eternally, meaning He's explaining how the Lord will respond to our mistakes while we live on earth. He's not speaking of how God will judge us after we die. We see this distinction when we consider the full implications of Jesus' comment and the context of His statement:

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. God's forgiveness of us is contingent on our forgiving others, but we know from other Scripture that believers 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.

Furthermore, if our choice to forgive another determined our eternal destiny, then Jesus would be preaching a Gospel of works. To forgive someone is a work, and yet we know the Bible clearly states that we are forgiven on the basis of God's grace and not by our works:

Eph. 2:8 For by grace you have been saved through faith; and that not of yourselves, it is the gift of God;
Eph. 2:9 not as a result of works, so that no one may boast.

Therefore, Scripture precludes us from interpreting Jesus' words as speaking of our eternal judgment. Instead, we must understand Jesus to be speaking in terms of earthly, temporal outcomes, not eternal outcomes. In other words, He’s not asking us to forgive someone from the penalty of their sin, for only God can do that. Jesus is asking us to be willing to restore relationships with those who have sinned against us and hurt us, for this is the good work we’ve been called to perform:

1Pet. 2:19 For this finds favor, if for the sake of conscience toward God a person bears up under sorrows when suffering unjustly.
1Pet. 2:20 For what credit is there if, when you sin and are harshly treated, you endure it with patience? But if when you do what is right and suffer for it you patiently endure it, this finds favor with God.
1Pet. 2:21 For you have been called for this purpose, since Christ also suffered for you, leaving you an example for you to follow in His steps,

Jesus promises that if a believer forgives in this way, God will likewise forgive us in the same way. That is, the Lord will forgive us of our offenses against Him, which broke our fellowship with God. Our eternal salvation was never in question since no believer can sin their way out of salvation, for 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 justified and forgiven the eternal consequences of sin (i.e., the Second Death) from the moment of our faith in Christ, therefore all mentions thereafter of receiving God's forgiveness during our life as a believer speak of the earthly consequences for sin. 

Nevertheless, our sin on earth may still bring the possibility of God's discipline and other earthly consequences. The Lord may choose to hand out earthly discipline when we sin against Him, and the Bible says the discipline of the Lord drives 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 to avoiding God's discipline is to repent, confessing our sins while 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.

But Jesus says that if we are so hardhearted we hold other believers' sins against them and break fellowship with them, then the Lord will likewise withhold His forgiveness of our sins. Instead, He may bring us earthly discipline. Our eternal salvation remains secure, but our earthly life will suffer. 

If we desire the Lord to forgive us of our daily missteps, we must be prepared to extend our forgiveness to other beleivers 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