As Professor John Lennox , from Oxford University, says "You cannot hold God responsible for all disease and our Lord Himself made that perfectly plain to His disciples, who had believed this view [suffering caused by divine retribution]. They came across a man blind from birth and the disciples asked Christ, “Who did sin? This man or his parents, that he should be born blind?”. They had attributed it, according to popular thinking, either to the man or to his ancestors. It was a kind of insipient doctrine of karma and Christ said, “Neither”. So I believe that it is false to say that all illness is a result of God’s discipline or anything else."
Professor John Lennox
If you believe the gospel, then God came down to earth and became a human being, suffered and died for us, out of love. He spent much of His time here healing people, as the lyrics of the Christmas song illustrate:
Mary, did you know that your baby boy
Would one day walk on water?
Mary, did you know that your baby boy
Would save our sons and daughters?
Did you know that your baby boy
Has come to make you new?
This child that you delivered, will soon deliver you
Mary, did you know that your baby boy
Will give sight to a blind man?
Mary, did you know that your baby boy
Will calm the storm with his hand?
Did you know that your baby boy
Has walked where angels trod?
When you kiss your little baby
You kiss the face of God
The blind will see, the deaf will hear
The dead will live again
The lame will leap, the dumb will speak
The praises of the Lamb
Mary, did you know that your baby boy
Is Lord of all creation?
Mary, did you know that your baby boy
Would one day rule the nations?
Did you know that your baby boy
Is heaven's perfect Lamb?
That sleeping child you're holding is the great, I Am
Jesus' arrival was predicted around 700 years before His birth, in the book of Isaiah (chapter 9) and speaks of His future reign in the Messianic Age:
Every warrior’s boot used in battle
and every garment rolled in blood
will be destined for burning,
will be fuel for the fire.
For to us a child is born,
to us a son is given,
and the government will be on his shoulders.
And he will be called
Wonderful Counsellor, Mighty God,
Everlasting Father, Prince of Peace.
Of the greatness of his government and peace there will be no end.
He will reign on David’s throne and over his kingdom,
establishing and upholding it
with justice and righteousness
from that time on and forever.
The zeal of the Lord Almighty will accomplish this.
God will make everything new, in the end:
"...knowing this first of all, that scoffers will come in the last days with scoffing, following their own sinful desires. They will say, “Where is the promise of his coming? For ever since the fathers fell asleep, all things are continuing as they were from the beginning of creation.” For they deliberately overlook this fact, that the heavens existed long ago, and the earth was formed out of water and through water by the word of God, and that by means of these the world that then existed was deluged with water and perished. But by the same word the heavens and earth that now exist are stored up for fire, being kept until the day of judgment and destruction of the ungodly.
But do not overlook this one fact, beloved, that with the Lord one day is as a thousand years, and a thousand years as one day. The Lord is not slow to fulfil his promise as some count slowness, but is patient toward you, not wishing that any should perish, but that all should reach repentance. But the day of the Lord will come like a thief, and then the heavens will pass away with a roar, and the heavenly bodies will be burned up and dissolved, and the earth and the works that are done on it will be exposed.
Since all these things are thus to be dissolved, what sort of people ought you to be in lives of holiness and godliness, waiting for and hastening the coming of the day of God, because of which the heavens will be set on fire and dissolved, and the heavenly bodies will melt as they burn! But according to his promise we are waiting for new heavens and a new earth in which righteousness dwells.
(2 Peter 3:3-13).
Why hasn't He stepped in before? I believe, as this passage indicates, that it is to delay judgement and give as many people as possible the chance of salvation.