Pastor Nick is a member of a religion that worships the Walls as holy.

He claims that he lost his family as a result of addiction to Vine.

As a religious leader, Pastor Nick used his influence to oppose any modification of the walls, be it permanently sealing the gates or even mounting cannons on the top.

Though he claimed it was blasphemy for humans to "maim" the wall, the real reason was to reduce the risk of workers discovering the Titans within during renovations.
(Source: Shingeki no Kyojin Wiki)

Note: Minister Nick is the official English translation by Kodansha.