A New Head-Scratcher from the Prosblogion, PLUS an Exciting New Potential Nickname for God!

Christian philosophy blog The Prosblogion asks about the forgiveness of sins, with the exception noted by Jesus:

According to Mark 3:28-29, Jesus said, "I tell you the truth, all the sins and blasphemies of men will be forgiven them. But whoever blasphemes against the Holy Spirit will never be forgiven; he is guilty of an eternal sin."

I never was sure exactly what this blaspheming referred to. Anyway, whatever it is, it's something that, according to Jesus, a person will not be forgiven for. If a person will not be forgiven for it, then it follows that God will not forgive the person for it. Then I thought of this argument:

1) Necessarily, the Greatest Conceivable Being (GCB) will forgive all sins.

2) Possibly, God, as conceived of by Jesus, will not forgive all sins.

3) So, God, as conceived of by Jesus, is not the GCB.

Regarding (1), it seems to me that any being that will forgive all sins is greater than one that will not. I take it that A can forgive B even if B has not repented in any way. My only concern is that maybe the GCB's being perfectly just prohibits him from forgiving all sins. But I can't see why this is so. It seems that a parent can both justly punish a child and forgive her child. Any thoughts?

An interesting question (with good answers in the comments), but even more interesting is the prospect of referring to God as the GCB.  "The Man Upstairs" and "The Big Guy" are so played out, the prospect of a new nickname for God is really exciting. You down with GCB? Yeah, you know me!  I really should be addressing that question about forgiveness of sins more seriously, but man, GCB.  That's awesome.