Without God, what is left of morality?
If you can only be moral-figure out right and wrong actions-if under the instruction of an authority figure, you are a philosophical child. People with mature consciences have no trouble with questions of morality in the absence of an authority figure.
Without God, what purpose is there in man's life?
Well, biologically, we have the same purpose as all other life on this planet--to reproduce to carry on the species. Now it is arguable that humankind has been a bit too successful in this purpose...
If we do not believe in God, how can we be certain of anything?
I can't figure out how anyone can feel certain of "god". Lot's of other things I AM certain about-the sun will come up in the morning, I would give my life for my children, the first Friday of the month is going to be busy at the bank where I work...
If God does not exist, whom can we turn to in a time of crises?
I've never known a god to step in and help out in any crisis. When I was a Witness his inability/disinclination to help out was always because he was "testing your faith"...
If there is no afterlife, who will reward virtue and punish injustice?
Uhhh...Virtue is its own reward. And injustice is not always punished. Geez...do you really believe that life, the cosmos and everything is FAIR??? Too bad you can't ask Job's first wife...
If God does not exist, what becomes of the worth and dignity of each person?
(See above) It never seemed that the god of the Bible gave much evidence of seeing the worth or dignity of each person...ya know like the first born of Egypt, or Job's first batch of children, or the people of Jericho, those NOT on the ark....
Without God , how can man achieve happiness?
That happiness thing was a much bigger struggle when I worried about Yahweh being in the mix...