Traditionally, the Catholic Church has been opposed to 'mixed marriages' between Catholics and non-Catholics. However when they do take place, it is considered desirable that the non-Catholic partner should receive teaching (instruction) in the essentials of the Catholic faith.
Instruction is also given to those who wish to convert to the Catholic faith. Evelyn Waugh received instruction from Father Martin D'Arcy* prior to being received into the Catholic Church in September 1930 (at the Church of the Immaculate Conception in Farm Street).
*This Jesuit priest is referred to in Muriel Spark's novel, The Girls of Slender Means, where it is said that one of the characters could never make up his mind between suicide and an equally drastic course of action known as Father D'Arcy.