I was a christian before I turned agnostic.
But if I was still a christian, I would say follow the example of
the good samaritan and help out of selflessness. Even if he was on
the vergoe of death and knowing that his eternal salvation or
eternal damnation lies minutes away. By doing so you may actually
save his life, even if you do not in the end, persuade him to the
side of christianity, you have acted justly.
Outside of this scenario, lets say you were lying in the middle of
the road dying your heart out and someone reasonably prosperous
comes up to you to preach. Would you accept this wholly unfamiliar
verbatim and take faith on the words of a stranger that refuses to
help you live?
But being agnostic that I am, I would feed and clothe him then turn
his gratitude into servitude. In that way, both our needs are
met.