Charles Ellicott Commentary John 4:18

"for thou hast had five husbands; and he whom thou now hast is not thy husband: this hast thou said truly." — John 4:18 (ASV)

In that you said truly.—The stroke goes deeper. It lays bare the secrets of all those years over which she thought the veil of the past had forever been drawn. The bright days of joy and dark days of sin; the heart’s promises made and broken; the sad days of death, which five times over had robbed her of a husband; or, worse than death, the sin which had severed the sacred bonds; the shame of the present shameless life—all these are at least hidden from a stranger.

But His words pierce to the innermost thoughts, and prove Him to know all the acts of her life (John 4:29). You have said well, A husband I have not. The holy name may not be given to the paramour you now have; with the loss of purity is linked the loss of truthfulness; the very truth you utter is meant to convey to Me an untruth, but to One who knows all, the words are really true: in that you said truly.