I was using the Ancestry.com census records when I found the Souders. 1880 - PA - Montgomery County - Hatfield Twp/District #8 If you are using another source, check for page 28.Lewis & Mary are lines 4 - 8 on that page.
1870 - PA - Montgomery County - Hatfield Twp For another source, look for page 29 (handwritten - upper left corner) or 424 stamped on the upper right. Lewis is on line 24.
Have you looked in the History of Souderton, compiled by Henry S Landes?The Souderton library has a copy - which is really a photocopy of the reprint that has been bound.I was able to find many of my ancestor's names mentioned with businesses to use as corroborating evidence.They also have a number of other unpublished manuscripts that cover some of the area cemeteries.That's what I used to find the tombstone info I posted earlier.There are several editions of Hatfield Township histories. The 1976 Update might be useful to you.
So far, I've used information that I've been able to find on-line as well as the Souderton Library.I haven't actually contacted any of the local Mennonite churches about their records.
One of the pastors in Hatfield is a genealogy buff.He has done some research on most of the families in his congregation.Although his congregation is not the Brethren, he has good ties with that church.The next time I talk to him I will ask him for you how to go about getting any info they might have.