simply believing that “Joseph Smith was a legitimate [and by implication, righteous] prophet; ergo, the things he taught are all necessarily true”
This makes sense to me. I remember being taught that the Book of Mormon is the keystone of our religion. That if I gained a testimony of it, then I had a testimony of Joseph Smith and all the prophets that have come after him in the LDS church.
This approach to gaining a testimony worked very well for me. I remember teaching it often to others on my mission and elsewhere.
It seems that having a testimony teaching by teaching would hedge up a testimony in that, each teaching would have effort placed into it to understand it and why it is true and what it means to me as an individual. Instead of a monolith testimony, you can have a network of testimonies. Like the Internet, if a node goes out, the network does not fail. And it is possible for the node to get brighter again as we gain knowledge and inspiration or a change of perspective.
I have done this teaching my teaching method more rigorously since I have had my experience of being shocked by unsavory details of the church history. I feel more grounded and strong in my network of testimonies.
It is not that we don't teach this kind of testimony building. We often say how we gained our testimony of tithing, or the word of wisdom or of temples. Maybe we just need to extend that to all parts of the gospel.
Our leaders have often said that we need to find out about the truth of the restored gospel. It is time that we shore up our networks of testimonies for the time that they will be tested further than we can conceive now.
Testimonies grow (and diminish) gradually.
Let us acknowledge that most often gaining a testimony is not a task of a minute, an hour, or a day. It is not once and done. The process of gathering spiritual light is the quest of a lifetime.-
If you seek God’s truth, that which now may appear dim, out of focus, and distant will gradually be revealed and clarified and become close to your heart by the light of God’s grace. Glorious spiritual vistas, unimaginable to the human eye, will be revealed to you. ("Receiving a Testimony of Light and Truth", Dieter F. Uchtdorf, Oct 2014)
Do you recall the slow and almost imperceptible increase in light on the horizon? In contrast to turning on a light in a dark room, the light from the rising sun did not immediately burst forth. Rather, gradually and steadily the intensity of the light increased, and the darkness of night was replaced by the radiance of morning. Eventually, the sun did dawn over the skyline. But the visual evidence of the sun’s impending arrival was apparent hours before the sun actually appeared over the horizon. This experience was characterized by subtle and gradual discernment of light.
From these two ordinary experiences with light, we can learn much about the spirit of revelation. ("The Spirit of Revelation", David A. Bednar, Apr 2011)