Any such testimony could be divided into two parts: why you left Mormonism and why you became Catholic. For me, I always begin by saying that I really loved being LDS, and even miss it sometimes (not the beliefs, more the social/community aspects).
I was raised Catholic, was a lector, extraordinary Eucharistic minister, etc. I was always curious about Mormonism, and had a Book of Mormon since high school (though I never read it). I took it with me when I went away to college, then to make a long story short, when I graduated and came back home, I decided to contact the missionaries. I was familiar with a lot of the LDS apologetics attempting to find LDS beliefs in “the ancient church”, an idea that would be part of why I went back to Catholicism a few years later. After contacting the missionaries, I was baptized 2 weeks later (I had already started reading the BoM on my own, and was pretty knowledgeable, and they were utterly surprised).
I enjoyed the LDS lifestyle, and my favorite thing was being around so many young people that were practicing their faith, and could talk about it. I liked having Institute, Sunday School, and other venues to study the scriptures and the Gospel. I especially loved the temple. Coming from Catholicism, Sacrament Meeting was sort of a step down for me. Maybe a few weeks after my baptism, I was ordained a priest in the Aaronic Priesthood (after which I blessed or passed the Sacrament each week), and then received my limited-use temple recommend to go with the ward to do baptisms for the dead. I loved it. I loved going to a sacred place where only other believers were. It was very beautiful inside, and I loved how everyone was wearing white, smiling, saying “welcome to the temple”, :D.
A few more months later, I was ordained an Elder in the Melchizedek Priesthood. At this point, I had already served as assistant secretary to the Elders Quorum Presidency, then ward missionary, helping the full time missionaries and teaching the Gospel Principles class. I still enjoyed it all, and didn’t really have any negative thoughts about anything. I stood up at testimony meeting all the time and said how I “knew” the church was true, and that it really was restored. At the year mark, I received my temple recommend to be Endowed. This was what I was waiting for. Finally I could see the rest of the temple, make more covenants, etc. I invited friends from the ward, the bishopric came as well, and the missionaries that baptized me came. As a former critic, I was already somewhat aware of what was going to happen, so I wasn’t surprised at the film, the tokens, the apron, etc. I enjoyed the experience, and really thought I was participating in something “ancient”. After being Endowed, I could be a proxy for all of the ordinances, administer some of them, and attended a friend’s sealing.
Shortly after, I received a call from the secretary or the clerk of the stake presidency (don’t remember which). I had to meet with the SP. I figured it was about a calling, and tried to think of what it could be. When we met, I was told that I was to be the next Elders Quorum President in my ward. I probably shed a few tears. After I was set apart and given keys for my calling, I did many things in the hope of strengthening the brethren, gave blessings, participated in setting aparts, confirmations, ordinations, etc.
It was at this point that I began to think about things that I had put on the mental “shelf”. A number of things were on that shelf, including:
-God the Father was once a man that progressed to Godhood
-the Great Apostasy
-Book of Mormon archaeology
-prophets that didn’t really seem like the Biblical prophets
But what really bothered me was the priesthood/temple ban of blacks. It just made absolutely no sense to me. Now, I used to say things like, “God restricted His priesthood during Old Testament times, so there is precedent, and I’m just glad we have the priesthood now”, but it made no sense in the context of Jesus Christ and what He came to do and did. Jesus Christ said to take the Gospel to all nations. Mormons did not go to black African nations specifically during this time. The nature of salvation history is one of progression, beginning with one couple, to families, nations, etc, leading up to Jesus Christ, who opened the fulness of Truth, Himself, to all. With the priesthood ban, black men could not participate in ordinances necessary for eternal life. Prophets of God said horrible things about blacks, including things related to spiritual conduct in the pre-mortal existence. These weren’t just personal, private opinions/thoughts, but declarations said from the pulpit, in books, magazines, etc. I couldn’t imagine being a member at that time. This sort of policy did not comport with my thoughts of what the Church of Jesus Christ should be doing, let alone what we read of that Church in the New Testament.