That's the flow - grace comes down to us for our sins and then it's meant to flow out to others for their sins.
If that's the pattern, what does it mean when we find another's sin "unforgiveable"? Well at that point we're accusing them of blasphemy. Why do I say that? Well that's what God calls the unforgiveable sin - blasphemy against the Holy Spirit. In Mark 3:28-30 we learn that rejection of Christ as Saviour is the blasphemy - it is the unforgiveable sin (for more on that, see here.)
But that language is interesting. When God says we've done something unforgiveable (i.e. finally and forever rejected Christ), He calls it a blasphemy. My point here is this: when we deem the sins of others to be "unforgiveable" we are saying that they have blasphemed. They haven't blasphemed the Spirit, they've blasphemed our god.
I'll explain it like this. We might well find ourselves in the position of knowing:
1) Christ has forgiven me,
2) Christ commands me to forgive, and that...
3) the offences against me are minor - not only relative to Christ's forgiveness but even when compared to other atrocities in the world.
But, it can still feel impossible to forgive. At that point we're deeming the offender to have committed an unforgiveable sin. In other words the offender has blasphemed our real god (our "functional saviour" to use a Tim Kellerism).
I might find countless offences to be "water off a ducks back" but if someone ruins my reputation, or if they harm my career or if they in any way hurt my children - that's unforgiveable. At those moments it's good to be aware that "unforgiveable" is synonymous with "sacrilegious." And it's good to identify the real god who we think is being blasphemed.
When the idol of "my reputation" or "my career" or "my family" is uncovered, it's actually a huge step forwards in forgiveness. Because now I'm confronted with the reality of my own need. I must repent and seek forgiveness. She may have ruined my reputation. But I worshipped it. When I confront the ugliness of my own blasphemy, my eyes are taken off the horizontal and fixed on the vertical. I realize I'm not so much "offended party" as "offender". In the language of Matthew 18, I start to realize the vastness of the ten thousand talent debt. And the 100 denarii becomes instantly relativized - not just in theory, but hopefully as a felt reality.
So here's my contention - maybe I'm wrong, correct me in the comments - but I reckon...
If there's something "unforgiveable" in my eyes, there's something blasphemous in my heart.