Prayer is calling upon a supernatural Person in faith expecting supernatural results according to the decisions and actions of responses of that supernatural Person.
The answer needs to come from the delegated sources offered by that supernatural Person, He will uphold and act upon the statements made according to the principles shown in Amos 3:7.
My answers can only give mortal opinions, not supernaturally upheld promises and decrees from Heaven.
Jesus prayed for the unity of faith, purpose, character, and action in His Church, in John Chapter 17, such unified prayer by a local SDA Church is in keeping with the spirit, attitude, principles, and actions spoken of in Jesus's prayer in John Chapter 17.
A local Church body amplifies and concentrates the sum of it's praying believing individuals, faith and trust, into the ears, mind, and heart of Jesus, through such, of one mind and heart unified prayers; like focusing concentrating sunlight through a focused magnifying glass on a bright clear day.
They (that local Church) are setting in motion two principles of Christ's teachings,
1. "That THEY may be ONE as WE are."
2. "according to your faith be it unto you."
As a church, we have received great light. This light the Lord has entrusted to us for the benefit and blessing of the world. To us has been given the ministry of reconciliation. With power from on high we are to beseech men to be reconciled to God. We are encouraged to pray for success, with the divine assurance that our prayers will be heard and answered. "If two of you shall agree on earth as touching any thing that they shall ask, it shall be done for them of My Father which is in heaven. For where two or three are gathered together in My name, there am I in the midst of them" (Matthew 18:19, 20). "Ask of Me, and I will answer your requests." {9MR 303.2}
The promise is made on condition that the united prayers of the church are offered, and in answer to these prayers there may be expected a power greater than that which comes in answer to private prayer. The power given will be proportionate to the unity of the members and their love for God and for one another.--Letter 32, 1903, p. 5. (To Brother and Sister Farnsworth, January 28, 1903.)
White Estate Washington, D. C. October 22, 1979 {9MR 303.3}
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("united prayer* " church = 7 hits ), from search engine query ( prayer church body= 30 hits ) I noted this one RE EGW and prayer for healing for someone she did not personally know the life experience of......, from
February 26, 1897 "Prayer for the Sick:"--........ {GCDB, February 26, 1897 par. 1 - 9}