Since my fellows SDA brothers are telling me that the Trinity its a mystery, therefore we cannot understand it, I decided to research a bit on how did this “mystery” entere the SDA church, because it wasn`t there in the beginnig.
Infact, the Pioneers we`re against the Trinity according with their own writings.

Trinitarian historians reveal that it took over 50 years and the death of the Pioneers in order for this doctrine to creep into the church.

“Second, the development of the Trinity doctrine demonstrates that
sometimes doctrinal changes require the passing of a previous generation. For Seventh-day Adventists, it took over 50 years for the doctrine of
the Trinity to become normative.”
History of Seventh-day Adventist
Views on the Trinity
Merlin D. Burt

Where did it start and where did it end?

The SDA movement became a church officially in 1863 (see more here)

This is what the founders believed about the Trinity:

“My parents were members of long standing in the Congregational church, with all of their converted children thus far, and anxiously hoped that we would also unite with them. But they embraced some points in their faith that I could not understand. I will name two only: their mode of baptism, and doctrine of the trinity.” Joseph Bates 1868

“The doctrine of the Trinity which was established in the church by the council of Nice, A. D. 325. This doctrine destroys the personality of God, and his Son Jesus Christ our Lord. The infamous, measures by which it was forced upon the church which appear upon the pages of ecclesiastical history might well cause every believer in that doctrine to blush.” – J.N. Andrews, Review & Herald, March 6, 1855,vol. 6, no. 24, page 185

“The doctrine called the trinity claiming that God is without form or parts; that the Father, Son and Holy Ghost, the three are one person, is another [false doctrine].” – Uriah Smith, Review and Herald, July 10th 1856, ‘Communications’

“Here we might mention the Trinity, which does away the personality of God, and of his Son Jesus Christ, and of sprinkling or pouring instead of being “buried with Christ in baptism,” “planted in the likeness of his death:” but we pass from these fables to notice one that is held sacred by nearly all professed Christians, both Catholic and Protestant. It is, the change of the Sabbath of the fourth commandment from the seventh to the first day of the week.” – Advent Review & Sabbath Herald, December 11, 1855, vol. 7, no. 11, page 85, par. 16

The Trinity was not the god of the founders of the church.
And the God they worshiped was the most important point of their faith, I would say the cornerstone.

“Many of our people do not realize how firmly the foundation of our faith has been laid. My husband, Elder Joseph Bates, Father Pierce, [Older brethren among the pioneers are here thus reminiscently referred to. “Father Pierce” was Stephen Pierce, who served in ministerial and administrative work in the early days. “Father Andrews” was Edward Andrews, the father of J. N. Andrews.—Compilers.] Elder [Hiram] Edson, and others who were keen, noble, and true, were among those who, after the passing of the time in 1844, searched for the truth as for hidden treasure. I met with them, and we studied and prayed earnestly. Often we remained together until late at night, and sometimes through the entire night, praying for light and studying the Word. Again and again these brethren came together to study the Bible, in order that they might know its meaning, and be prepared to teach it with power. When they came to the point in their study where they said, “We can do nothing more,” the Spirit of the Lord would come upon me, I would be taken off in vision, and a clear explanation of the passages we had been studying would be given me, with instruction as to how we were to labor and teach effectively. Thus light was given that helped us to understand the scriptures in regard to Christ, His mission, and His priesthood. A line of truth extending from that time to the time when we shall enter the city of God, was made plain to me, and I gave to others the instruction that the Lord had given me. ” 1SM p 206.4

These people were very serious in their research and study, and the god you worship its a big deal.

So how did we get to have the Trinity as a Fundamental point of doctrine?

Some are “blaming” E.G. White for that and bring couple of quotes like “the third person of the godhead” or “unborrowed, underived life”, but she never mentioned the word Trinity in those texts, or god the Son or god the holy spirit.

The facts are very simple but the change was so smooth that the later liders hide it throughout the history, hence the title.

The steps, as I said are very simple:
As the movement started to grouw after the great disappointment, the new converts came from other churches with their own baggage of doctrines and they weren’t so diligent in study as the pioneers.
And like today, the youth despises the elderly and labels them old school, but they cannot change anything until the seats of power are vacant.

The first major attempt that I found in bringing the Trinity doctrine on the table is the “secret” Bible Conference from 1919.
I’m saying secret because it wasn’t opened to everyone, but only by invitation, and the transcript of the discussions we’re locked into a safe for a very long time, but this is another subject for another day.

Three pastors we’re advocating for the Trinity but without success.

E.G. White died in 1915. Four years later they’ve made an open attempt, but unsuccessful because “the old timers” we’re still in “power”.

In 1926 the General Conference is commissioning Leroy Froom to “give a series studies” on the doctrine about the personality of the holy spirit.
The man himself admits to be empty handed from the pages of “spirit of prophecy” and turns toward the “harlot/Babylon” for inspiration.

“May I here make a frank personal confession? When, back between 1926 and 1928, I was asked by our leaders to give a series of studies on the Holy Spirit, covering the North American union ministerial institutes of 1928, I found that, aside from priceless leads found in the Spirit of Prophecy, there was practically nothing in our literature setting forth a sound Biblical exposition in this tremendous field of study. There were no previous pathfinding books on the question in our literature. I was compelled to search out a score of valuable books written by men outside of our faith—those previously noted—for initial clues and suggestions, and to open up beckoning vistas to intensive personal study. Having these, I went on from there. But they were decided early helps. And scores, if not hundreds, could confirm the same sobering conviction that some of these other men frequently had a deeper insight into the spiritual things of God than many of our own men then had on the Holy Spirit and the triumphant life. It was still a largely obscure theme.” – Movement Of Destiny. p. 322