One of the membership vows new United Methodists take is to support the church with their witness. Hal Knight explained why we seek to do this (and that “why” is not the answer usually given) in an article that I’ve been meaning to share for a long while. Here’s an excerpt the summarizes the article:
the primary argument for the gospel is lives and churches who in their relationships, their life together, and their outreach to others is motivated and characterized by love. It is both the reason for and the result of evangelism. 
I encourage you to head over and listen to the whole thing.
 Henry H. Knight, “Great Commandment Evangelism,” Catalyst Resources, April 6, 2016, section goes here, accessed December 01, 2017, http://www.catalystresources.org/great-commandment-evangelism/.