A sign is a thing brought openly to view, containing in itself the manifestation of something hidden, as the sign of Jonas represents the descent to hell, the ascension of Christ, and His resurrection from the dead. Hence it is added, For as Jonas was a sign to the Ninevites, so shall also the Son of man be to this generation. He gives them a sign, not from heaven, because they were unworthy to see it, but from the lowest depths of hell; a sign, namely, of His incarnation, not of His divinity; of His passion, not of His glorification.
Reproduced in full. Translation: Oxford 1841 (Newman), public domain (Public domain).
Also cites: in Esai. 7.