The challenge of the witness to keeping the message pure - Acts 15v1-21

How does the witness keep his message pure, fresh, alive and clear?
What are the subtle threats to the content and quality of messages of the witness?  The church down the ages has battled with this.  Paul and Barnabas did in Chapter 15.  Martin Luther fought the Catholic Church on account of this problem.  Denominations of the Christian faith have come up as a result of responses to these issues.