I've puzzled over this for quite some time. Its one thing to accet the fact, but another to fathom the reasoning behind it. I finally came up with this. If God is the Father, the Son and The Holy Spirit, then God crucified himself for us! The only difference between God and Jesus was that Jesus was God made flesh. If God is all-powerful, totally good etc, He could have no idea what it is like to be human and have the frailties of humans for example jealousy, hatred, the power of temptation on us and why we give in. By becoming human He experienced all this for Himself, a sort of understanding of the human condition - a sort of bridge between the human and the Divine. Now God can use this bridge to have a personal relationship with all mankind, He's more sympathetic of our weaknesses and more able to forgive us.
Another idea which I came up with was based on a peculiar line in a version of the Creed. It goes
".....He suffered death and was buried. He descended into Hell. On the third day He rose again....."
If Satan demands payment for all our sins, could God have been paying for them either himself or by giving His Son to suffer the torments for us? I was told that this was a heresy since it implied that God was beholden to Satan in some way.
Anyway, I think the key to understanding it all is to remember that jesus and God are one and the same.