Question: I should know the answer to this already, but why do Christians think there can never be more revelation from God (as opposed to the Mormons, who are just fine with God revealing more ideas and are willing to give them equal weight to the Scriptures)? I am assuming we base our idea on “do not add or take away from the Scriptures”. Is that it?
Answer: Not necessarily. It is more, I think, that our experience has been that nothing anyone else has offered as prophetic or Scripture (since the apostles) has equaled the apostles’ work and it has been the church’s experience that everything we need we have found in the New Testament and Old Testament. Or, as in the case with “scriptures” like the Latter Day Saints have produced, we have found them in contradiction to our Bible.
We believe that God orchestrated getting into our canon (our accepted list of books) all that we needed for formulating our doctrinal understanding of the gospel and who God is. We certainly respect the doctrinal traditions of the church and writings of other important church leaders, but we see them more as witnesses to the final authority of Scripture and guides to its interpretation. Anything that claims to be Scripture must pass muster with the existing Scriptures, and none so far have passed that test.
This does not mean that there haven’t been and might still be revelations that speak to particular needs. It is more doctrinal revelation that seems to have ceased or been made unnecessary. We do not equate the other kinds of revelations with Scripture since they are more time bound or related only to one specific situation.
The Revelation 22:18,19 warning seems related specifically to the book of Revelation, not the Scriptures generally. You might note that the same kind of warning is given in Deuteronomy 4:2 and 12:32, and Scriptures have surely been added since then.