Oh no! Not New Advent, too!

http://www.newadvent.org/library/almanac_thisrock94.htm
http://www.catholic.com/thisrock/1994/9410qq.asp

You stated that when the Latin Mass says Christ's blood was shed "pro multis," normally rendered "for many," it can equally be translated "for all" since "many" is a biblical idiom that often means "all" (cf. Dan. 12:2, Rom. 5:12). If so, why was this not stated for 1900 years? Also, did "for many" always mean "for all" in the Bible?

It has been stated for 1900 years that Christ's blood was shed for all. The Church declared heretical the Calvinist/Jansenist idea that Christ shed his blood exclusively for the elect. The translation "for all" may not have been used in the liturgy until recently, but this was because in the Western rite the Mass was celebrated in Latin until recently. There were no English translations made for liturgical use.

Today there is still variation among the different language translations of the Mass. In some languages, such as Spanish, the rendering of "for many" has been kept, while in others, such as Italian, the rendering of "for all" has been used. In other rites of the Church there are further variations on the words of consecration. The Greek term for "many" in the consecration (pollus) is not always an idiom for "all." In Mark 5:26 we are told that the woman with the issue of blood had suffered under many (pollus) physicians, but she certainly had not been to all the doctors in the world. Here the term simply meant, "many." In the case of the words of consecration, we know that "all" is a valid translation because the Bible tells us Christ did shed his blood for all men (1 John 2:2), and the Church has condemned as heretical the contrary proposition.


My commment:
It has not been stated for 1900 years that "many" is a biblical idiom for "all". As you note, the Mass was celebrated in Latin until recently. There we say that Christ said his blood would be shed "pro multis" (for many), not "pro omnibus" (for all). Given that Catholic Bibles uniformly state that our Lord said He would shed His blood "for many", it is unlikely that such a statement is heretical.