Jamieson, Fausset and Brown on John 4:25-26

25-26. I know Messias cometh ... when He is come, &c.--If we take our Lord's immediate disclosure of Himself, in answer to this, as the proper key to its meaning to His ear, we can hardly doubt that the woman was already all but prepared for even this startling announcement, which indeed she seems (from Joh 4:29) to have already begun to suspect by His revealing her to herself. Thus quickly, under so matchless a Teacher, was she brought up from her sunken condition to a frame of mind and heart capable of the noblest revelations.

tell us all things--an expectation founded probably on De 18:15.