Truthfulness involves two virtues: the virtue of Sincerity and the virtue of Accuracy (I borrow Bernard Williams' terminology). One should speak what one believes to be the truth, but one should also strive to discover the truth. A person who (intentionally) fails to do the former is a liar, but a person who is gulled by a lie often fails in the duty of Accuracy. It is no excuse for making false statements that one was misled by someone else--if these lies were within one's power to detect. Sincerity is not enough in the goal of speaking the truth. You can be culpable for being complicit in a lie, even when you don't know that it is a lie.