Full Idea
The Jesuits rejected occasionalism ... because it is incompatible with the Catholic interpretation of the Eucharist, which there is genuine change of substance of the bread into the substance of Christ (transubstantiation).
Clarification
In the Eucharist, bread becomes the body of Christ
Gist of Idea
Occasionalism contradicts the Eucharist, which needs genuine changes of substance
Source
Richard T.W. Arthur (Leibniz [2014], 5 'Substance')
Book Reference
Arthur, Richard T.W.: 'Leibniz' [Polity 2014], p.122
A Reaction
Not sure I understand this, but I take it that the Eucharist needs a real relation across the substance-spirit boundary, and not just a co-ordination.