A person can be sincere and still be mistaken.

Honesty and accuracy are separate things. Someone can believe every word they say, have no intent to mislead, and still be wrong, because sincerity is about the relationship between a person and their words, not between their words and the world. Trusting a claim because the speaker clearly means it is a common mistake, and it is often made about the people we like most.

When someone you trust tells you something important, let the trust settle the question of their intent and not the question of the facts. Ask how they know, gently and without suspicion. Good-faith people rarely mind the question, and the answer tells you whether you are receiving knowledge or a strongly held impression.