I absolutely truly believed "with every fiber of my being". People often try to negate a person's "loss of testimony" by saying that "they never really had a testimony in the first place". They say this because it is less threatening to them. They think to themselves, "I could never fall like that person because my testimony is too strong".
Anyone who ever knew me before my apostasy would have to confess that I really did believe probably more than most members, so if I could come to believe it is all false now, then that should shake them that there really might be something to what I claim to have learned about the Church.