Idiomatic expression for being totally off in ones statement/belief

Consider,

out in left field Ngram

Slang Completely mistaken; wrong

Also, out of left field. Eccentric, odd; also, mistaken. For example, The composer's use of dissonance in this symphony is way out in left field, or His answer was out of left field; he was totally wrong. This idiom refers to baseball's left field but the precise allusion is disputed

Random House

You could also consider using off-base which means:

Wrong, relying on a mistaken premise, as in His description of the accounting system was totally off base. This metaphoric term originated in baseball, where a runner who steps off a base can be put out. [c. 1940]

[The American Heritage® Dictionary of Idioms]

Fig. unrealistic; inexact; wrong (Typically: be off-base; get off-base.): I'm afraid you're off base when you state that this problem will take care of itself. You're way off base if you think I was to blame!

[McGraw-Hill Dictionary of American Idioms and Phrasal Verbs. © 2002]

You can add adverbs such as totally and way before it to intensify the idiom.