"God is not to be measured by our understanding of what is just. We have to be the first to admit that our understanding of virtually everything is somehow warped and twisted and affected by our own sinfulness.
In Psalm 50 and verse 21, God said, "You thought that I was altogether like you." And certainly He is not. In Isaiah 55 and verse 8, "My thoughts are not your thoughts, neither are your ways My ways, declares the Lord, for as the heavens are higher than the earth, so are My ways higher than your ways and My thoughts than your thoughts." And there is the key. God has ways and thoughts that are to us incomprehensible, unresolvable, inscrutable. There is a great benediction in the eleventh chapter of Paul's letter to the Romans in which he says this, verses 33 to 36, "O the depth of the riches both of the wisdom and knowledge of God. How unsearchable are His judgments and unfathomable His ways, for who has known the mind of the Lord, or who became His counselor?" Who could know how God thinks? Who could be so bold as to tell God how He ought to think? "For from Him and through Him and to Him are all things, to Him be the glory forever. Amen."
It is an essential understanding of God that He is holy, that His nature is holy, that He is infinitely and perfectly just, that He is morally flawless and perfect. That He is perfection. Everything in Him and of Him and for Him and from Him and by Him is perfect. And so whatever He says is just, is what justice is."
~John MacArthur
Taken from this sermon:
http://www.gty.org/resources/sermons/90-273/the-doctrine-of-election-part-1