I believe you're being earnest. I'm taking what you're saying as what you mean. When someone says "I don't believe X" it means they don't believe it. When so

I believe you're being earnest. I'm taking what you're saying as what you mean. When someone says "I don't believe X" it means they don't believe it. When someone says "I don't necessarily believe X" it means they kinda do believe it. Not one person who believes the world is spherical would ever say (or agree that it correctly captures their view) the statement "I don't necessarily believe the world is flat". [EDIT: I thought this example might help make things clearer. version 1: - wife to husband: you're spending a lot of time with that new co-worker. Do you find her attractive? - husband: no, I don't find her attractive. & version 2: - wife to husband: you're spending a lot of time with that new co-worker. Do you find her attractive? - husband: no, I don't necessarily find her attractive. A husband that genuinely did not find the co-worker attractive would never express things in the second way. It's clear in that second way that he does find her attractive to some extent but doesn't want to say it] Sometimes "I don't necessarily believe..." is used when the person feels awkward admitting that they have some belief, or when they haven't consciously realised that they do. It can be when they haven't fully admitted it to themselves.