Testimony and reality of experience should be the final test, not the first, for the accuracy of a theory. In every discipline of the sciences and humanities, (if done properly), inquiry precedes conclusion. If this is not true of an inquiry, you have based the conclusion on a presupposition, and therefore your conclusion is discredited. When done properly, a theory is postulated, and then it is tested. The conclusion is based on how the theory corresponds to reality.
When debating against Christianity, the debater cannot simply point to all sorts of atrocities, tragedies and distortions of Christianity to disprove it. You must look to what the Bible really says, and how it plays out in the life of someone who actually follows the Bible. To challenge Christianity you can’t merely challenge those who were not in line with it, just as I couldn’t question the reliability of modern science by quoting someone who believes the earth is flat, or challenge the merits of psychology by quoting a Scientologist.
When defending Christianity, my testimony is what brings the philosophical and theological to life. The Bible is reliable, historically and factually. I have sound reasons for believing in God. My life is a testimony that what the Bible says is not only theory, but reality. In short, testimony is the conclusion of the study of the whole. If not, then testimony becomes merely a subjective opinion that can be challenged and easily refuted by an equally subjective opinion. Yes, testimony does hold powerful ground in and of itself, telling of how the power of God has been displayed in your life. But to the skeptic it holds no ground unless it is the conclusion of the matter, as opposed to the starting point.