Please answer a question about the following article about Pope Paul VI:

Between the third and fourth sessions the pope announced reforms in the areas of Roman Curia, revision of Canon Law, regulations for mixed marriages involving several faiths, and birth control issues. He opened the final session of the council, concelebrating with bishops from countries where the Church was persecuted. Several texts proposed for his approval had to be changed. But all texts were finally agreed upon. The Council was concluded on 8 December 1965, the Feast of the Immaculate Conception.

What does the Catholic church considered "mixed" in a "mixed marriage"?
faiths