Green’s Dictionary of Slang

souper n.1

[such conversions, however nominal, were often achieved by the appeal of Protestant missionaries handing out free soup, orig. at the time of the great famine of 1845–7]

1. (orig. Irish) a convert from Roman Catholicism to Protestantism; thus any Protestant; thus souping n., conversion fron Roman Catholicism to Protestantism.

[UK]Sl. Dict. 302: Souper an Irish Roman Catholic who pretends conversion ― or perversion ― so as to obtain a share of the soup and blankets provided for Protestants only by Christian missionaries. These recalcitrants are also called ‘swaddlers’.
[UK]Daily News 20 Jan. 6/4: They cannot believe in any Catholic honestly becoming a Protestant. The convert must be a souper .
[US]Monteleone Criminal Sl. (rev. edn) 218: souper [...] an insincere convert; one who works the churches to advance himself.
[Ire]L. Daiken Out Goes She 16: Proddy Proddy ring the bell / Call the soupers back from Hell.
P. Sommerville-Large Coast of West Cork 78: The question of souping is a delicate and emotional one; many proselytising clergymen have been unjustly accused of this unpleasant form of pressure .
[Ire]M. Ryan et al. No Shoes in Summer n.p.: He [my grandfather] [...] was originally RC but changed to C of I during the Famine not because he was a ‘souper’ as was common practice then, but because he didn’t agree with the Pope’s infallibility .

2. one who scrounges free soup tickets.

[UK]Farmer & Henley Sl. and Its Analogues.