"How on earth can Paul affirm that God does what He forbids others to do; that He does what He says He will Himself never do; that He does it habitually and that He even designates Himself, 'the God who justifies the wicked' or (we might say) "who righteous's the unrighteous"? It is preposterous! How can the righteous God act unrighteously and so overthrow the moral order, turning it upside down? It is unbelievable!
Or rather it would be, if it were not for the Cross of Christ. Without the Cross, the justification of the unjust would be unjustified, immoral and therefore impossible. The only reason God "justifies the wicked" (Romans 4:5) is that "Christ died for the wicked" (Romans 5:6). Because He shed His blood in a sacrifical death for us sinners, God is able to justify the unjust".
John R W Stott - "BST Series - The Message of Romans" - IVP, Leicester (1994) - (p112-3).