The phrase "hook, line, and sinker" is often used to signify a total acceptance or belief in something. A synonym for this phrase could be "completely fooled," indicating a high level of gullibility or naivety. Another synonym could be "completely sold," meaning that the individual is fully convinced or confident in what they are believing. Additionally, the phrase "without reservation" could be used as a synonym, implying a total lack of doubt or hesitation in one's acceptance of an idea or statement. Finally, "wholeheartedly" could also be considered a synonym, indicating a complete, enthusiastic adoption of a belief or point of view.