The Jewish religion provides a separate Monotheist witness to the truth of the Catholic religion. Islam provides a similar type of testimony.
And, of course, you have the testimony of the Church Herself in Her profession of faith.
The Protestants are, ex professo, just bad Catholics, a witness by defect of the Catholic faith.
Non-believers are simply bad men, because the first commandment requires all men to worship the one true God. "I am the Lord your God..." that is the first commandment. God must be your Lord. They also, therefore, are a witness by defect of the Catholic faith.
"Catholic" means the religion of/for all men.