Seventh-day Adventists say the “Testimony of Jesus” in Revelation 14:12 is Ellen White's writings. What does the Bible say the Testimony of Jesus is? It's the Gospel.
1 Timothy 2:6 : “…who gave Himself as a ransom for all, the testimony given at the proper time.”
1 Corinthians 1:6 : “…even as the testimony concerning Christ was confirmed in you.”
2 Thessalonians 1:10 : “…when He comes to be glorified in His saints on that day, and to be marveled at among all who have believed — for our testimony to you was believed.”
2 Timothy 1:8 : “Therefore do not be ashamed of the testimony of our Lord or of me His prisoner, but join with me in suffering for the gospel according to the power of God.”
1 John 5:11-12 : “And the testimony is this, that God has given us eternal life, and this life is in His Son. He who has the Son has the life; he who does not have the Son of God does not have the life.”