Definition: an incredulous or habitually doubtful person
Doubting Thomas comes from the name of St. Thomas, the apostle who doubted the resurrection of Jesus. The name has been used to refer to an incredulous person for a number of centuries, dating back at least to the late 17th century. Thomas was not the only apostle to lend his name to an idiomatic term; Judas may be found in Judas goat, the name for an animal (often used figuratively) who leads other animals to a slaughterhouse or similarly unfortunate destination.
To make yet a doubting Thomas believe the existence of no Popry in this codicle, let him thrust his hand into the side and looke upon the Prints of these things, in the body of the Church Reformed, he shall, or may be ascertained of its Innocency….
—William Annand, Mysterium Pietatis, 1671