I am a volunteer teaching English in my church but I have many illiterate adults trying to learn English, Is it better to teach them first to learn how to read and write in their native language?

Good question?

I would guess no.

I don't see how learning to read and write in a different language will be of benefit to learning to read and write in English. I may be wrong.

I guess it would seem to me you are just doubling what needs to be learned, since they already don't know how to read and write in their own language, I'm not sure how learning to do that, will make it any easier to learn in English.

I have to agree with prognur, it would seem like an added burden to them, not an actual help.

I am of German decent, although I have lived in Canada most of my life, and have spoken English most of my life. I cam to Canada at the age of 3 1/2 years old, German was my native tongue, but of course, at that age, I could not write or read in German. I came to Canada, went to school in Canada, learned to read and write in English and then learned to read and write in German, on my own. It might be the same for these people, although it is easier to learn when you are a child.

Good luck, let us know how it goes. :)

Yes. I think you teach them some English words and as you progress you start showing them how to write those words and read the same words and keep building on it. It will be slow going but not as slow as trying to teach reading and writing in the native language and then trying to change everything over to English.
If I was from France and illiterate but had migrated to USA, teaching me to read and write French would be counterproductive because sentence formation is quite different and would hinder me later on trying to read and write English. I could be all wet, n'est pas?