Opinionated

"That's only your opinion" is not a valid statement when your so-called opinion is based on the opinion, teachings, science, or doctrine of someone else. If the 'only your opinion' statement is addressed to you, and you know your opinion is authentically and accurately based on someone, or some group's established facts, then your response can be, "No, it's not just my opinion, it's what ______ also believes, teaches, and researched."