"Whomever does the good, even if they do not yet recognize God explicitly, is the mediator of the goodness of God," he said. "For us grace and nature are belonging together, and are not in a contradiction. Grace and nature, faith and reason, must be distinct (but) not separate."
Cardinal Mueller also exhorted those in attendance to be on guard against totalitarian ideologies that set themselves up against the vision of God and the Church.
Many ideologies of the 20th century were totalitarian in that they sought to establish their own vision of creation, a "humanism without or humanism against God," he said.
Forms of totalitarianism exist today, he explained, like "the concept of designing one's baby," and the promotion of "euthanasia for those who are too tired to live, or who have become un-useful up to the sociological laboratories that want to make humanity happy with their political and economic theories, but in reality only enslave to their fantasy for omnipotence."
Christians must fight injustices in the world in the name of human dignity, he insisted.
"Nobody may divert their gaze while the number of souls who go hungry grow, are deprived of their rights and recused to slavery, while the trauma of the refugees arriving on the European shores and the American border increase, and while being in a unified world, the risks and the challenges of globalizations are ever present," he said.
"At the foundation of this dignity," he added, "are the rights to lodging, food, and clothing, as well as the right to earn a living for himself and for the well-being of his family."
The cardinal also warned against what Pope Francis has called the "ideological colonization" of the developing world, where developed countries try to force programs like abortion, birth control, sterilizations, and the approval of same-sex marriage onto developing countries.
Cardinal Mueller called this "an aggressive importation of a deformed image of the human person of the so-called 'society of well-being'."
These "developing cultures" cannot be ignored or trampled underfoot, he said, as "variety enriches," which is also "the message of Pentecost when all peoples in diverse languages announce together the great works of God in the language of love."
Christians, he insisted, must not only confess God with words, but work to do His will "by holding faithful to the Gospel and benefitting from its resources."
(Story continues below)
Subscribe to our daily newsletter
"Not everyone who cries or confesses 'Lord, Lord,' will enter the Kingdom of Heaven, but those who do the Father's will by taking a strong grip on the work at hand," he continued. "Now the Father wills that in all men we recognize Christ our brother and love Him effectively in word and in deed."
"By thus giving witness to the truth, we will share with others the mystery of the Heavenly Father's love," he concluded.
"As a consequence, men throughout the world will be aroused to a lively hope, the gift of the Holy Spirit, that some day at the last they will be caught up in peace and utter happiness in that fatherland radiant with the glory of the Lord."