The first known use of the term “the catholic church” (note the small “c”) was about 110 A.D.; in a letter from an early Christian leader, Ignatius, to the [Church of the] Smyrnaeans:
“…even as where Jesus may be, there is the katholike ekklesia“…katholike ekklesia is translated “universal church” or “the catholic church” or “the one and only church”.
For the full text of the letter, click here. All Christians can enjoy reading what Christian leaders of the generation following the Apostles wrote.
By the beginning of the 4th century, the descriptive (adjective) “catholic” became the popular name (noun) of the Church of the Christians: The Catholic Church.