The article on "Testimony" in the LDS doctrinal guide True to the Faith (TTTF) is a nice supplement to my earlier post on this topic (see also this thoughtful response by Geoff). The article defines testimony as "a spiritual witness given by the Holy Ghost," which "usually comes as a quiet assurance, without spectacular displays of God’s power." I think that understates the power of the Mormon method, which seems capable of developing a testimony without any display of God's power!
According to the article, as a Mormon you have the "opportunity and responsibility" to get a testimony, and a "duty" to keep it. In fact, your "happiness in this life and throughout eternity" depends on it. The results can be "miraculous and life-changing." So you can imagine how disappointed your fellow believers will be if you are so foolish as to maintain the position "I believe, but I'm not really comfortable saying I know." They drum that nonsense out of you at the MTC: You're asking people to change their lives and make a big commitment, Elder. They want to hear that you really know, not just believe. Of course you know; why else would you be here serving a mission? Just say you know, and over time that knowledge will come to you. That's right out of the TTTF article: "Your testimony will increase as you share it."
I don't object to the notion that faith is a gift from God that some have in greater proportion than others, or that faith is an amalgam of hope and experience. I just object to the cavalier but mandatory use of the terms "knowledge" and "knowing" to describe the LDS mindset. At the least, it's fair to maintain that "spiritual knowledge" is of a different sort than the fact that three angles of a triangle sum to 180 degrees or that it rains more in Seattle than in Phoenix (respectively, examples of deductive and empirical knowledge).
Reading through the TTTF article, it almost sounds like they are describing commitment rather than testimony: No one receives a complete testimony all at once. Your testimony will grow stronger through your experience. It will expand as you show your willingness to serve in the Church, wherever you are called. It will increase as you make decisions to keep the commandments. ... Your testimony will increase as you share it.
No doubt some readers have a different conception of spiritual knowledge. Let's hear it.