Seventh-Day Adventism, or the doctrine held by Seventh-Day Adventists (who sometimes call themselves “Adventists”), believe generally in the full gospel of Jesus according to the Bible. Appropriately, they recognize that God is spirit, and that in addition to the written word He reveals Himself to individuals directly by His spirit. But in addition to adding some extraneous, extrabiblical details to their doctrines, some of those details are in non-conformance to Scripture, and as such are heretical. God’s revelation to individuals by His spirit will never be in conflict with the scripture of the Bible.
Ironically, Seventh-Day Adventists claim to agree that revelation cannot conflict with scripture. Sola scriptura. But they don’t. Their favorite prophetess, Ellen G. White, made a large number of anti-biblical doctrinal statements of error, and her teachings are heralded as extensions to scripture. This wouldn’t necessarily be wrong (but would be extremely dangerous) if her writings weren’t scripturally conflicted. And those writings conflict with scripture severely.
A video by Allen Parr (not affiliated) explains his own findings regarding Seventh-Day Adventism. Click the link below.