John Gill Commentary John 4:48

"Jesus therefore said unto him, Except ye see signs and wonders, ye will in no wise believe." — John 4:48 (ASV)

Then said Jesus to him With some degree of roughness in his speech, and severity in his countenance, in a way of reproof for his unbelief, as if he could not heal his son without going down to Capernaum along with him:

except you see signs and wonders you will not believe. This was the cast of the Jews everywhere, both in Judea and Galilee; they required signs and miracles to be wrought, in confirmation of Christ's being the Messiah, and which indeed was but right; and Christ did perform them for that purpose: but their sin of unbelief lay in this, that they wanted still more and more signs; they could not be contented with what they had seen, but required more, being sluggish and backward to believe.

Our Lord seems to say this chiefly for the sake of the Galilaeans who were about him. Though they might be acquainted with his former miracles when among them (of turning water into wine) and had seen his wondrous works at the feast at Jerusalem, they were yet very desirous of seeing more, and perhaps very pressing for this cure.