Mysterium Fidei §15

¹⁵ First of all, We want to recall something that you know very well but that is absolutely necessary if the virus of every kind of rationalism is to be repelled; it is something that many illustrious martyrs have witnessed to with their blood, something that celebrated fathers and Doctors of the Church have constantly professed and taught. We mean the fact that the Eucharist is a very great mystery—in fact, properly speaking and in the words of the Sacred Liturgy, the mystery of faith. "It contains within it," as Leo XIII, Our predecessor of happy memory, very wisely remarked, "all supernatural realities in a remarkable richness and variety of miracles." (4)