We love your letters! We try to share them and how your experiences have helped your testimony grow. Thank you for your example.
Mennonites are a Christian sect closely related to the Amish. They believe in a simple way of life and believe that by giving up certain things they are becoming closer to Christ.
They do believe that certain modern inventions are witchcraft (though they are not as nearly extreme as the Amish). They believe by abstaining from those things they are avoiding unnecessary temptations from the "gentiles".
My Aunt was a Mennonite. She met my Uncle while serving a mission for her sect. He was on his Mission and well, he testified and the Spirit bore witness. After his Mission he had my Grandma and Grandpa drive him out to Illinois to meet the family and get there blessing to get married. She was baptized and they married in the Temple a year later.
Now it has been something like 40 years and they are still happy as ever.
I think the key to teaching anyone is to be " Challenging and Testifying" (Alvin R Dyer - great talk given in the 70's). There are those who recognize and feel the Spirit. They are those who will accept the invitation and for those who do not - well, to sum up Elder Dyer, we can check the later at judgment. Our only responsibility is to give them the opportunity.
The Mennonites are a dedicated people, but the Spirit can bear witness of the truth when we testify of Christ, Joseph Smith, and the Book of Mormon to them. Remember, according to Elder Packer, that a testimony can be found in the bearing of it, so invite those you teach to share their feelings - then recognize when the Spirit bears witness of the truth they share. Once you know they felt the Spirit, it is then up to you as a representative of Christ to Challenge them. Challenge them to read the Book of Mormon, come to Church, say a Prayer, or Repent and be Baptized.
I know you are about the Lord's work and you will be blessed. Have faith in your testimony and know that among the many prayers offered in your behalf, we pray for you too. You will be blessed and you will see many receive the truth and come closer to Christ - no matter what faith they are. Even if they are Mennonites.