Read his ‘Credo of the People of God,’ in his famous apostolic letter Solemni hac liturgia, given motu proprio. He lamented (in the 1960s) the uncontained desire for novelties among several influential Catholics, which was causing great danger (‘disturbance and perplexity’) to the Faithful, even challenging the very basics of the Catholic Faith such as the Sacrament of the Eucharist.
So he sought to make an act of faith on our behalf. This is the part about the Blessed Sacrament. The full version of the letter is here.
“Christ cannot be thus present in this sacrament except by the change into His body of the reality itself of the bread and the change into His blood of the reality itself of the wine, leaving unchanged only the properties of the bread and wine which our senses perceive. This mysterious change is very appropriately called by the Church transubstantiation. Every theological explanation which seeks some understanding of this mystery must, in order to be in accord with Catholic faith, maintain that in the reality itself, independently of our mind, the bread and wine have ceased to exist after the Consecration, so that it is the adorable body and blood of the Lord Jesus that from then on are really before us under the sacramental species of bread and wine, as the Lord willed it, in order to give Himself to us as food and to associate us with the unity of His Mystical Body. The unique and indivisible existence of the Lord glorious in heaven is not multiplied, but is rendered present by the sacrament in the many places on earth where Mass is celebrated. And this existence remains present, after the sacrifice, in the Blessed Sacrament which is, in the tabernacle, the living heart of each of our churches. And it is our very sweet duty to honor and adore in the blessed Host which our eyes see, the Incarnate Word Whom they cannot see, and Who, without leaving heaven, is made present before us.”
Apostolic letter Solemni hac liturgia of the Holy Father Paul VI, June the 30th, 1968 [link]
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