"Thus will the truth be brought into practical life, and thus will be answered the prayer of Christ, uttered just before His humiliation and death: “That they all may be one; as Thou, Father, art in Me, and I in Thee, that they also may be one in Us: that the world may believe that Thou hast sent Me.” John 17:21 . The love of Christ, the love of our brethren, will testify to the world that we have been with Jesus and learned of Him. Then will the message of the third angel swell to a loud cry, and the whole earth will be lightened with the glory of the Lord. 6T 401.1"
"That they all may be one; as thou, Father, art in me, and I in thee, that they also may be one in us: that the world may believe that thou hast sent me."
Ellen White directly references John 17:21 in this passage.