You could also ask the question, are Methodists, Episcopal, Baptists Christians?
For I know persons in different denominations and they call themselves "Baptist" etc, but are NOT saved, for salvation is ONLY by an encounter with the Lord Jesus, having faith in Him, washed in Hs Blood and being born again! Acts 2v21, 4v12, John 3v16, Eph 1v7, 2v8.
So, I think a more appropriate question would be, can a person be saved and "belong to" the 7th Day Adventist church?