Studying the Scripture through self-study and group-study as well as personal research on other faiths has taught me many things. Including the old axiom "In essentials unity; in non-essentials, liberty; in all things, charity."
Check out chapter six from the book
Reasons for Faith by Norman Geisler.
It talks about these biblical "essentials" for salvation that are held by the orthodox (lower case o ) church.
Hence I believe Mormons are in error because they believe baptism by immersion is necessary for salvation.
taken from an address to the Harvard Divinity School in March 2001 by Robert L. Millet, former dean of religious education at Brigham Young University.
What Mormons Believe About Jesus Christ - LDS Newsroom
To be honest, there website is very vague. I do not find that they adequately flesh out their beliefs and instead ask you to pray about it, which in itself is fine, if you know what to ask God about!
7th day Adventists? I know several. I believe them to be Christians as far as I know. I disagree with their doctrine on the Sabbath. Other than that, I haven't really studied them too much.