Here:

Our sins are called ‘debts’ because they leave us accountable to God, and freedom from them can only come by way of forgiveness. Thus ‘in the first section of the prayer the highest perfection is set before us, but in the latter our weakness’. Those who truly aspire to the glory of God in all things are most aware of their own needs. The addition of the words ‘as we forgive our debtors’ is not meant to make our receiving forgiveness conditional on our granting it, but to show the freeness and wonder of God’s grace. If we can willingly forgive others, how much more does God freely forgive us? As Calvin comments on Matthew 6:12: ‘if the Spirit of God reigns in our hearts, every description of ill-will and revenge ought to be banished’.

Here is the apparently unbloggable Calvin:

Wherefore, we are not to ask the forgiveness of our sins from God, unless we forgive the offenses of all who are or have been injurious to us. If we retain any hatred in our minds, if we meditate revenge, and devise the means of hurting; nay, if we do not return to a good understanding with our enemies, perform every kind of friendly office, and endeavour to effect a reconciliation with them, we by this petition beseech God not to grant us forgiveness. For we ask him to do to us as we do to others. This is the same as asking him not to do unless we do also. What, then, do such persons obtain by this petition but a heavier judgment?

Modern Presbyterianism is about as good at remembering Calvin as a politician is at remembering his campaign promises.

See here for the Reformed understanding of conditions in our relationship with God.