Apologetics SEBTS Bruce A. Little, PHD
Paraphrase.....
One's testimony of what Christ has done for one personally only gives a sense of certainty to oneself. It is not portable. This certainty is not transferred to the hearer. Experience cannot be transferred and cannot be used as a part of a persuasive argument in evangelism.
When Paul gave his testimony to Felix it was not to convince Felix of the truth of Christianity but as to why the charges against him were false.
Beliefs that we hold on too quickly can often be given up quickly. Forcing the issue of salvation often leads to spurious confessions of faith.