Albert Barnes on John 4:39-42

And many of the Samaritans of that city believed on him, &c. There is seldom an instance of so remarkable success as this. From a single conversation, in Circumstances, in a place, and with an individual little likely to be attended with such results, many sinners were converted; many believed on the testimony of the woman; many more came to hear, and believed because they heard him themselves. We should never despair of doing good in the most unpromising circumstances, and we should seize upon every opportunity to converse with sinners on the great subject of their souls' salvation.

(a) "for the testimony" Jn 4:29