What is the difference between "Pro Bono" and "Pro Deum"?

Answer 1

I may have found the source of the confusion as to where the phrase “pro deum” came from.

In line 4 of the Catholic prayer “Ora pro nobis Deum” Pray for us to God

There may have been an error where the nobis was deleted. Such errors are occasional found in manuscripts. Just as in line 1 the word caeli has been turned into the coeli.

The Reginal Caeli (Latin)

Regina coeli, laetare, alleluia.

Quia quem meruisti portare, alleluia.

Resurrexit sicut dixit, alleluia.

Ora pro nobis Deum, alleluia.

V. Gaude et laetare, Virgo Maria, Alleluia,

R. Quia surrexit Dominus vere, alleluia.

Oremus: Deus qui per resurrectionem Filii tui, Domini nostri Iesu Christi, mundum laetificare dignatus es: praesta, quaesumus, ut per eius Genetricem Virginem Mariam, perpetuae capiamus gaudia vitae. Per eundem Christum Dominum nostrum. Amen.

Answer 2

"Pro bono" means "on behalf of the good" and is usually used in the context of doing something for free or for the benefit of society.

"Pro deum" doesn't really mean anything because the preposition pro takes the ablative case, and deum is either the accusative masculine singular of "deus" which means God, or the genitive masculine plural of the same word. Grammatically it doesn't work.