Be watchful, the bishops of the Catholic world are advised, against certain trends:
203. But in all these matters, it is essential that you watch vigilantly lest the enemy come into the field of the Lord and sow cockle among the wheat;[Cf. Matt. 13:24-25] in other words, do not let your flocks be deceived by the subtle and dangerous errors of false mysticism or quietism – as you know We have already condemned these errors;[Encyclical letter Mystici Corporis] also do not let a certain dangerous “humanism” lead them astray, nor let there be introduced a false doctrine destroying the notion of Catholic faith, nor finally an exaggerated zeal for antiquity in matters liturgical. Watch with like diligence lest the false teaching of those be propagated who wrongly think and teach that the glorified human nature of Christ really and continually dwells in the “just” by His presence and that one and numerically the same grace, as they say, unites Christ with the members of His Mystical Body.
Perhaps some bishops feel beset by various difficulties. I have no idea what those might have been in different corners of the world just a few years after WWII–aside from the need to rebuild civilization in Europe. A word from the Ash Wednesday Lectionary:
204. Never be discouraged by the difficulties that arise, and never let your pastoral zeal grow cold. “Blow the trumpet in Sion . . . call an assembly, gather together the people, sanctify the Church, assemble the ancients, gather together the little ones, and them that suck at the breasts,”[Joel 2:15-16] and use every help to get the faithful everywhere to fill the churches and crowd around the altars so that they may be restored by the graces of the sacraments and joined as living members to their divine Head, and with Him and through Him celebrate together the august sacrifice that gives due tribute of praise to the Eternal Father.
“Every help” was, in part, the liturgical reform long desired and advocated even sixteen years before Sacrosanctum Concilium. With this paragraph, the main portion of Mediator Dei ends. Two more posts to come: an epilogue.