Lutheran pastor, Johannes Brenz (1499–1570), writes in his Commentary on John:
It is not a true faith which is conceived from miracles and wonders or signs. For miracles are merely seals and witnesses. A seal certifies nothing new except what has already been committed to writing. In the same way, witnesses say nothing new, but they bear witness to things which have been heard or seen. Thus, miracles do not confer faith, and they do not take something uncertain and make it certain, but they confirm something that is already certain. They do not make believing persons out of unbelieving persons, but they confirm already believing persons in their faith.
This is why Jesus sometimes reproves people who ask him to do miracles or show them signs.