Why Seventh-days Adventist only know a portion of this doctrine.
It would be foolish of us to believe that Satan does not want SDA’s to be lost. When the pioneers and Mrs. White were alive, God gave them our foundational doctrines including the truth of the Godhead, all founded on unquestionable authority. Satan's success in leading some astray in the early days of Adventism was minimal. But he began working on a plan to deceive the majority of the denomination after the pioneers and God's prophet passed from the scene and on this point he has been very successful. Evangelists, educators, pastors and those in leadership positions along with the laity have been deceived, despite Jesus’ warnings about deception in the last days in Matthew 24.
A misunderstanding of The Seal of God and the Mark of the Beast could cause an SDA to be worshiping a false God. Do you think that is impossible? Nader Mansour from Australia lays it out as plain as day in the below video and you will see where we do not have the complete picture and correct understanding of this doctrinal truth. I suggest taking notes as you will be surprised at how Satan has brought most of us under his control, when we think we have just what we need to know. Prepare to be shocked.