There is nothing to add to the message of the Gospel. The purity of the message of Christ must be protected and left as clear as the Scriptures make it.

Galatians 2:4–5
[4] Yet because of false brothers secretly brought in—who slipped in to spy out our freedom that we have in Christ Jesus, so that they might bring us into slavery—[5] to them we did not yield in submission even for a moment, so that the truth of the gospel might be preserved for you. (ESV)