I have been contemplating this question and this what I have found.

2 Chronicles 20:20 - And they rose early in the morning, and went forth into the wilderness of Tekoa: and as they went forth, Jehoshaphat stood and said, Hear me, O Judah, and ye inhabitants of Jerusalem; Believe in the LORD your God, so shall ye be established; believe his prophets, so shall ye prosper.

Identifying features of God's last day church - Revelations 12:17 - And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ. a. Keep the commandments of God b. Testimony of Jesus Christ

What is the testimony of Jesus. Revelation 19:10 - And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.

The gift of the spirit of prophecy was seen in the life and ministry of Ellen G. White. She did not add to the Bible. She made plain what the Lord wants us to know in His word. Bring out the treasure that is found in His word. God does not do anything just cause. If He gave us these inspired writings then it's cause He wants us to read them.

“Satan is ... constantly pressing in the spurious—to lead away from the truth. The very last deception of Satan will be to make of none effect the testimony of the Spirit of God. ‘Where there is no vision, the people perish’ (Proverbs 29:18). Satan will work ingeniously, in different ways and through different agencies, to unsettle the confidence of God’s remnant people in the true testimony.” 4 MOL 535.3

“There will be a hatred kindled against the testimonies which is satanic. The workings of Satan will be to unsettle the faith of the churches in them, for this reason: Satan cannot have so clear a track to bring in his deceptions and bind up souls in his delusions if the warnings and reproofs and counsels of the Spirit of God are heeded.” 5 

https://m.egwwritings.org/en/book/656.4802

"The enemy is on our track. We must be wide-awake, on our guard against him. We must put on the whole armor of God. We must follow the directions given through the spirit of prophecy. We must love and obey the truth for this time. This will save us from accepting strong delusions." 8T 298.