What is Frontier Missions?
Dr. Ralph Winter’s 1974 address shaped a lot of the modern American frontier paradigms. https://s3.amazonaws.com/files.frontierventures.org/pdf/TheHighestPriority.pdf
A followup by R.W. Lewis from 2018 about clarifying what the Frontiers are now, https://joshuaproject.net/assets/media/articles/ijfm_35_4-lewis.pdf