forgiveForgiving others is THE WAY to have forgiveness from God, and is A SIGN of that forgiveness:

1.  It is the way to have forgiveness. If ye forgive men their trespasses, your heavenly Father will also forgive you’ (Matthew 6:14).  But one would think other things should sooner procure forgiveness from God than our forgiving others.

No, surely nothing like this to procure forgiveness; for all other acts of religion may have leaven in them. God forbade leaven in the sacrifice (Exodus 34:25). One may give alms, and there may be the leaven of vainglory in it. The Pharisees sounded a trumpet, when they gave alms, to gain applause (Matthew 6:2).

One may give his body to be burned, yet there may be the leaven of false zeal in this; but to forgive others that have offended us can have no leaven in it, no sinister aim. It is a duty wholly spiritual, and is done purely out of love to God; hence God annexes forgiveness to this rather than to the highest and most renowned works of charity which are cried up in the world.

2. It is a sign of God’s forgiving us. It is not a cause of God’s forgiving us, but a sign. We need not climb up into heaven to see whether our sins are forgiven: let us look into our hearts, and see if we can forgive others. If we can, we need not doubt but God has forgiven us. Our loving others is the reflection of God’s love to us.

Oh, therefore, by all these arguments, let us be persuaded to forgive others. Christians, how many offences has God passed by in us! Our sins are innumerable and heinous. Is God willing to forgive us so many offenses, and cannot we forgive a few? No man can do so much wrong to us all our life as we do to God in one day.

Thomas Watson, Body of Divinity: Fifth Petition of the Lord’s Prayer