Pius XII warns against contemporary philosophical and theological errors that weaken the certainty of truth, revelation, dogma, and the Church’s teaching authority. He rejects historicism, existentialism, idealism, materialism, and an eirenism that seeks unity by minimizing essential doctrinal differences. Theology may study modern theories and acknowledge genuine scientific findings, but it must remain grounded in Scripture and Tradition as authentically interpreted by the Church, and it must preserve established concepts rather than surrendering to unstable systems. The encyclical commends Thomistic philosophy as a reliable formation for priests and insists that papal teaching, even when not exercising extraordinary infallibility, demands religious assent once it settles a question. In biblical interpretation, it defends divine authorship and inerrancy, including the historical truth conveyed through the metaphorical language of Genesis 1–11. Qualified scholars may investigate human evolution if they affirm the immediate creation of the soul by God, but polygenism is rejected as incompatible with original sin’s transmission from Adam. Bishops, religious superiors, and Catholic teachers are instructed to prevent these errors from spreading in schools, writings, conferences, and pastoral instruction.