In his book, Center Church, Tim Keller writes:
The popular saying “Preach the gospel; use words if necessary” is helpful but also misleading. If the gospel were primarily about what we must do to be saved, it could be communicated as well by actions (to be imitated) as by words. But if the gospel is primarily about what God has done to save us, and how we can receive it through faith, it can only be expressed through words. Faith cannot come without hearing. This is why we read in Galatians 2:5 that heresy endangers the truth of the gospel, and why Philippians 1:16 declares that a person’s mind must be persuaded of the truth of the gospel. Ephesians 1:13 also asserts that the gospel is the word of truth. Ephesians 6:19 and Colossians 1:23 teach that we advance the gospel through verbal communication, particularly preaching (pg 32).