Preaching recently on Matthew 18:21-35 - and particularly Jesus' words 'So also my heavenly Father will do to every one of you, if you do not forgive your brother from your heart' - I used this illustration:
When I give folks from outside Bournemouth directions to
our house I'll often say ‘Approach Bournemouth on the 338, then turn right onto the 3060 signposted Wimborne. If you get as far as Pizza Hut, you've gone too far. Pizza Hut is the sign you've gone wrong.' This text (said I) is the Pizza Hut on the road to hell!
For while some otherwise good commentators (Wiersbe, for example) miss the point of this passage by arguing it's not about salvation at all, the real meaning is just this: if being forgiven hasn't changed your nature so that you are a forgiving person, then you've not been forgiven at all.