take at word

To accept what one says without further verification or investigation. Why some people take that pundit at his word is beyond me. He clearly has an ulterior motive. You're right to be wary, but, in this case, I think we can take John at his word. He's just trying to help.

To be convinced of another's sincerity and act in accord with his or her statement: We took them at their word that the job would be done on time.

To believe someone, to regard someone as trustworthy. This locution dates from the sixteenth century, appearing in such sources as Miles Coverdale’s translation of the Bible (1535) and several of Shakespeare’s plays (e.g., “I take thee at thy word,” Romeo and Juliet, 2.2). It also is part of an amusing proverb quoted in David Ferguson’s Scottish Proverbs (1595) and numerous later collections: “Take a man by his word, and a cow by her horne.”