Chairity wont allow atheist to help.

[John V]

A "lie by omission" is a misrepresentation of fact when the failure to say something or to provide complete information would lead a reasonable person to an incorrect conclusion.

As defined by Yahoo


But since most christians don't seem to be reasonable, well who knows?

Interestingly you're lying by omission by posting this definition, as it's unlikely it was the first hit on your search. My google search had it at third. The first was:
http://en.wikipedia.org/wiki/Lie#Lying_by_omission
Also known as a continuing misrepresentation, a lie by omission occurs when an important fact is left out in order to foster a misconception. Lying by omission includes failures to correct pre-existing misconceptions. When the seller of a car declares it has been serviced regularly but does not tell that a fault was reported at the last service, the seller lies by omission. It can be compared to dissimulation.

You probably skipped this one because, if one has no intention of proselytizing, then their beliefs are not important to volunteering at a soup kitchen.