Roman Catechism 18.109

Chapter 18 — Part II: The Sacraments — The Holy Eucharist

Section 109. No less of caution should be observed by pastors in explaining the mysterious manner in which the body of our Lord is contained whole and entire under the least particle of the bread. Indeed, discussions of this kind should scarcely ever be entered upon. Should Christian charity, however, require a departure from this rule, the pastor should remember first of all to prepare and fortify his hearers by reminding them that no word shall be impossible with God.