In Matthew 22:17-21, Jesus said, "Give to Caesar what is Caesar's, and to God what is God's."
Is this what you are referring to? Some people read that and say that Jesus was saying it's OK to pay taxes. And that by his saying so we should pay "what we owe Ceasar" even if the government is corrupt. Which was the case with the Romans. Some would say that applies to the US government too.
Personally, I have no problem paying taxes. I do take note that because the Romans did collect taxes they had the tools necessary to capture, torture and crucify Jesus.
My view is that the government should not be in the charity business. I think it's a better idea for this to be handled by individuals. For example, supposedly 100' s of millions of people watched or were involved in Live 8 yesterday. A few bucks from each person would have gone a long way. But no, instead of working on the problem themselves a bunch of rich folks organized something that's goal is to shake down others. They used the Jesse Jackson model. I didn't see them referring to the Bible. And if they played and Christian music I missed it.
Oh, and another thing, charity should come from those that can afford to be charitable. The US is hardly in the position to be giving away billions since when we do so we are doing so using credit cards.
All this government charity reminds me of Amway. It seems like magic, but at some point someone has to sell the soap and in our case someone has to pay the taxes for all this kindness. There is no free lunch, well, not unless you were one of the performers yesterday or one of the people that will get our tax dollars to "help" the poor.
Bottom line? If you wish to point to the Bible as the basis for the government taxing me and then giving the proceeds (supposedly) to third world countries count me out.
It's a bit more subtle than that. Jesus was asked by a Pharisee whether or not it was "lawful" to pay taxes, as a way of trapping him - if he said no, he'd have the Romans on his case, if yes, he could be arrested by the Pharisees for sanctifying what they considered to be an illegal tax. He simply asked his questioner, Whose face do you see on this coin? "Caesar", came the obvious answer. Well, then let Caesar have what's his and give God what's God's.
In other words, don't use his coins if you don't like paying his tax. Just give them all back to him. (they didn't want to do that of couse, because they liked the convenience of his coins; they just didn't want to pay the tax that went along with using them)
It's not in the Constitution, but it's in the other writings this country treasures. The bible.
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Thou Shalt not steal is also in the bible. Charity has no purpose in Government:
http://www.neoperspectives.com/foundingoftheunitedstates.htm
The government of the United States is a definite government, confined to specified objects. It is not like state governments, whose powers are more general. Charity is no part of the legislative duty of the government.
James Madison
I feel obliged to withhold my approval of the plan to indulge in benevolent and charitable sentiment through the appropriation of public funds. ... I find no warrant for such an appropriation in the Constitution.
Grover Cleveland
I cannot find any authority in the Constitution for public charity.
Franklin Pierce