Ignatius (disciple of John) referred to the "Catholic Church" (in ~107 AD) as the visible, unified body of believers under episcopal authority, distinguishing it from heretical groups lacking that structure. In his Letter to the Smyrnaeans, he equates its presence with Christ's, implying a concrete institution rather than a vague universal idea. The term functioned as a proper descriptor for the apostolic Church, not just an adjective.
The Epistle of Ignatius to the Smyrnaeans - Chapter 8. Let nothing be done without the bishop;
"See that you all follow the bishop, even as Jesus Christ does the Father, and the presbytery as you would the apostles; and reverence the deacons, as being the institution of God. Let no man do anything connected with the Church without the bishop. Let that be deemed a proper Eucharist, which is [administered] either by the bishop, or by one to whom he has entrusted it. Wherever the bishop shall appear, there let the multitude [of the people] also be; even as, wherever Jesus Christ is, there is the Catholic Church. It is not lawful without the bishop either to baptize or to celebrate a love-feast; but whatsoever he shall approve of, that is also pleasing to God, so that everything that is done may be secure and valid."