"Why, if you don't believe in God, would you give up anything for Lent?"
I believe Christianity is not inherent to much of what it claims: the values, the parables, the idea of rebirth...The Good Life is not contingent upon a belief in God.
The story of the temptations of Christ is one with which I easily identify: avoiding temptation while in a weakened state, proving mind over matter leading to a rebirth. There is power in achievement; believing theoretically that you can meet a challenge is different than having the internal knowledge that you've met that challenge. It is easy to say that the mind is powerful, but to rise up to a trial and summon a renewal of confidence that comes from such a triumph over self can prove that your mind is more powerful than your body, your urges, your weaknesses.
All this for a 40 day test? Yes, please. Bring it ON!