marks of the Church
The Church is a means necessary for all men unto salvation.
Now there are many societies which claim to be the Church of God.
Hence, since there is an obligation imposed on men to enter the Church of Christ, this obligation presupposes the possibility of distinguishing the true Church of Christ from all other societies falsely claiming this prerogative.
The Church is materially and formally visible.
Hence there must of necessity be something in the Church which visibly manifests this society to be the true Church of Christ.
This visible sign is what we call a mark of the Church.
We may define a mark to be an essential characteristic which is proper to the true Church alone, and visibly manifests it to be the Church of Christ.
The Church itself points to four such marks in the Creed promulgated at the Council of Constantinople (381).
These marks are: One, Holy, Catholic and Apostolic.
Although theologians have at times multiplied the number of marks which distinguish the Church, they are reducible to these four.
The Catechism of the Council of Trent, published in 1566, with the highest official sanction, adopted this more simple arrangement of the four marks.
Today this arrangement has won universal acceptance.
New Catholic Dictionary