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1. To have confidence or faith in (a person), and consequently to rely upon, trust to.
a. To believe in a person (also in Scripture in, or on, his name). [Cf. late L. credere in aliquem.]
b. To believe in a thing, e.g. the truth of a statement or doctrine; also in mod. usage, in the genuineness, virtue, or efficacy of a principle, institution, or practice.
c. Formerly with of = on, in.
d. absol. To exercise faith.
2. To give credence to (a person, or his statement); to trust (from L. credere alicui). Obs. Replaced by 5, 6.
3. ellipt. To believe in (a person or thing), i.e. in its actual existence or occurrence.
4. To trust, expect, think to do (something). Obs. Cf. BELIEF 5.
5. To give credence to (a person in making statements, etc.). Object orig. dat.: cf. 2. Phrases. I believe you, an expression of emphatic agreement; believe (you) me, phr. strengthening an assertion.
6. a. To give credence to, to accept (a statement) as true [cf. L. credere aliquid]. Also in colloq. phrases strengthening an assertion, as believe it or not, would you believe it? (see WILL v.1 43), you'd better believe (see BETTER a. 4b).
b. To accept (a thing) as authentic. Obs.
7. With clause or equivalent inf. phrase: To hold it as true that..., to be of opinion, think.
8. To hold as true the existence of. Obs. (Now expressed by 3.)
Oxford English Dictionary.