If God can do anything, why was the cross needed?
Genesis 6:11 - 13
Now the earth was corrupt in God's sight and was full of violence. God saw how corrupt the earth had become, for all the people on earth had corrupted their ways. So God said to Noah, "I am going to put and end to all people, for the earth is filled with violence because of them. I am surely going to destroy both them and the earth."
What followed is the familiar, Sunday school, story. Noah and his family were blameless in God's sight and therefore didn't deserve to die. They built an ark and were saved from the world-wide flood that followed.
When the waters had dried sufficiently for the ark to reach land, Noah and his family disembarked. God spoke to him and made the following promise:
"Never again will I curse the ground because of man, even though every inclination of his heart is evil from childhood. And never again will I destroy all living creatures, as I have done."
Even faced with man's inclination towards doing the wrong thing, God has promised never to destroy the earth as He did with the flood. This effectively writes off the idea that He should "wipe the slate clean" and start from scratch again (much like He did with the flood).
So, simply destroying everything - the demonstrated, just, payment for the wrongdoing of man - isn't an option. Another method is required to deal with the matter. God is holy and utterly just and requires that justice be served. From the beginning, He is shown to not let justice go, in the sense of simply saying "Oh, it doesn't matter": the books must balance properly on this one.
The cross marks another judgement point very much like the flood. For several thousands of years people had been keeping the law of God, given through Moses, to one degree or another. By that point it was written into the very fabric of their society yet they still didn't do it right, even when so well practised.
God once again laid bare His anger at the wrongdoing of mankind. Once again, life was utterly obliterated. This time though, payment was being made by one man - Jesus of Nazareth, God's one and only son. Being true to the justice, God was judging wrongdoing. Being true to His love for mankind, it was being done sacrificially on our behalf by Jesus.
So, if God can do anything, why not simply wipe us all off the face of the earth and end it all?
"Do I take pleasure in the death of the wicked?" declares the sovereign Lord, "rather, am I not pleased when they turn from their ways and live."
"For I take no pleasure in the death of anyone," declares the sovereign Lord, "repent and live!"
"As surely as I live," declares the sovereign Lord, "I take no pleasure in the death of the wicked, but rather that they turn from their ways and live."
Given that God wants mankind to "turn from their (wicked) ways and live" He will, to be self consistent, act according to his statements and promises. Hence the cross. Jesus chose to lay his own life down so that justice could be done and yet give mankind the chance to turn from their ways and to live.
2 Peter 3:9
The Lord is not slow in keeping his promise, as some understand slowness. He is patient with you, not wanting anyone to perish, but everyone to come to repentance.
God is consistent and orderly, with the power to do anything. It isn't a matter of "He chose" in the sense of it being a fun thing to be doing, more that in the depth of His being he is orderly (just as the author is male - it's a thing that he can't chose to do / not do!) This means the God is being consistent with his Holiness and love in seeing the crime paid for while still giving the criminal the chance to repent and live (taking no pleasure if they chose not to.)
This, substitutionary offering of the cross can either be rejected or accepted by the person concerned. If accepted, the slate is wiped clean making the person right with God, just as the earth was through the flood, opening up an whole new era of relationship with Him.