We need the visible Church. There are many truths that men can discover on their own, but they can never discover most of those truths that are necessary for salvation. It’s not that these truths are hidden, but simply that they are beyond the powers of mortal men ever to see. We cannot discover on our own, for example, that God is a Trinity. This is not a truth that could ever by attained by human reason alone. But in His mercy God did not leave us in the dark. He has revealed those things that we need to know.
The Roman Catechism says that besides the fact of divine revelation, we also need an authorized teacher of that truth:
It is true that the invisible things of God from the creation of the world are, as the Apostle teaches, clearly seen, being understood by the things that are made: his eternal power also, and divinity. But the mystery which hath been hidden from ages and generations so far transcends the reach of man’s understanding, that were it not made manifest by God to His Saints, to whom He willed to make known by the gift of faith, the riches of the glory of this mystery among the Gentiles, which is Christ, man could by no effort attain to such wisdom.
But, as faith comes by hearing, it is clear how necessary at all times for the attainment of eternal salvation has been the labour and faithful ministry of an authorised teacher; for it is written, how shall they hear, without a preacher? And how shall they preach unless they be sent?
And, indeed, never, from the very creation of the world, has God, most merciful and benignant, been wanting to His own; but at sundry times and in divers manners spoke to the fathers by the prophets, and pointed out to them in a manner suited to the times and circumstances, a sure and direct path to the happiness of heaven. But, as He had foretold that He would give a teacher of justice to be the light of the Gentiles, that His salvation might reach even to the ends of the earth, in these last days he hath spoken to us by his Son, whom also by a voice from heaven, from the excellent glory, He has commanded all to hear and to obey. Furthermore, the Son gave some to be apostles, and some prophets, and others pastors and teachers, to announce the word of life; that we might not be carried about like children tossed to and fro with every wind of doctrine, but holding fast to the firm foundation of the faith, we might be built together into an habitation of God in the Spirit. [Roman Catechism, Introduction; emphasis added]
The fact that preachers must be sent (Romans 10:14, quoted above in bold) implies that we need a visible Church. Without a visible Church, we have no way of knowing whom God has actually sent to preach, because there is no objective means for judging a man’s claim to have been sent by God. We need to be able to judge such claims because we are faced with a world beset by preachers of all sorts who make contradictory claims and who also claim to have been sent by God to preach. How shall we judge among them? Who is right? We need an objective standard. For reasons that I’ve explained elsewhere, we need more than a bare appeal to the Bible. We need the certain guidance of the Church that Christ founded. We need the Catholic Church.