Not change the rules, but fulfill the rules.

There is sin in this world, a just God would punish all sin, even the slightest imperfection was not worthy of God, but humans are imperfect beings, capable of both good and bad on a whim, to subject all of humanity would be cruelty. And God is goodness, and love, so God turns himself into a man, instead of all of humanity being subjected to hell, he suffers hell instead, on the cross where he is humiliated and spat on by the men he sought out to save, lifted up for all to see, the God man dies, seemingly defeated, but in reality he had defeated the devil and his plot in eden to seperate God and the seed of adam for all eternity. On the 3rd day he rises up from the tomb, being the only man to do so, thereby defeating death as a man. In his resurrection, it proves that all who seek out and follow him will be lifted up from the grave and be with him even beyond the end of time.

God basically used a loophole against his own rule, not changing it, its not a cheap loophole by any means(which would cheapen it), since he suffered immensely. He was beaten, spat on, humiliated by crowds of people, whipped and scourged, sleep deprived, starved, cold, naked, scared shitless, forced to carry an absurdly heavy piece of wood around the city and up a hill, nailed to it for all to see and jeer at. Not a cheap price at all.