Brigham Young relates that Oliver Cowdery did not deny his testimony even after apostatizing.
Brigham Young, "Faithfulness and Apostasy," April 6, 1855, Journal of Discourses, 26 vols. (Liverpool: F. D. Richards, 1855), 2:257–258
Do you not know others who had manifestations almost equal to those Joseph had, but who have gone by the board? Martin Harris declared, before God and angels, that he had seen angels. Did he apostatize? Yes, though he says that the Book of Mormon is true. Oliver Cowdery also left the Church, though he never denied the Book of Mormon, not even in the wickedest days he ever saw, and came back into the Church before he died. A gentleman in Michigan said to him, when he was pleading law, “Mr. Cowdery, I see your name attached to this book; if you believe it to be true, why are you in Michigan?” The gentleman read over the names of the witnesses, and said, “Mr. Cowdery, do you believe this book?” “No, sir,” replied Oliver Cowdery. “That is very well, but your name is attached to it, and you say here that you saw an angel, and the plates from which this book is said to be translated, and now you say that you do not believe it. Which time was you right?” Mr. Cowdery replied, “There is my name attached to that book, and what I have there said that I saw, I know that I saw, and belief has nothing to do with it, for knowledge has swallowed up the belief that I had in the work, since I know it is true.” He gave this testimony when he was pleading law in Michigan. After he had left the Church he still believed “Mormonism;” and so it is with hundreds and thousands of others, and yet they do not live it.