Have some doubts
misgiving: a feeling of doubt or apprehension about the outcome or consequences of something.
qualm: an uneasy feeling of doubt, worry, or fear, especially about one’s own conduct; a misgiving.
acquiesce: accept something reluctantly but without protest.
demur: raise doubts or objections or show reluctance.
dissent: the expression or holding of opinions at variance with those previously, commonly, or officially held. Also disagree, oppose, disapprove, object means to disagree.
Disagree seriously and loudly
denounce: publicly declare to be wrong or evil.
remonstrate: make a forcefully reproachful protest.
expostulate: express strong disapproval or disagreement.
rebuke: express sharp disapproval or criticism of (someone) because of their behavior or actions.
There are a lot more words about disagreement and each of them has a little distinct shade to it.