Those who proclaim themselves to be the sole measure of realities and of truth cannot live peacefully in society with their fellow men and cooperate with them. (CSDC §142)
Truth is one because reality is one; a given proposition is true to the extent that it conforms to what is. Consequently when men differ over truth, they are differing over more than trifles. They find themselves at odds over the way the world really is.
Now how can two such men live together peacably in the long run? The splintering of societies and fellowships amply testifies to the difficulties of these disagreements.
This is why it is so important for Christians to agree about doctrine, and why our failure to do so has spawned the fractured Christianity of our day. If we cannot agree about the truth, we cannot agree about the way things really are. If we cannot agree about them ourselves, why should any non-Christian believe anything that we say about Jesus?
We do not have the final say about the truth. Our consciences do not rule the day when it comes to how the world is. It’s the exact opposite: we have to think about the world rightly by conforming our thoughts to the way the world really is, and when it comes to supernatural things — the stuff of theology and revelation and dogma — the only way that we can hope to be in the truth is by believing what God has revealed (since by definition the supernatural transcends our natural powers of discernment).
We do not get to decide what God has said; I do not stand as an arbiter of supernatural truth. For this we need the help of the Church, to which God has vouchsafed His revelation. When we accept the Church’s authority about supernatural things we can be at one in our beliefs. There is no other possible means for unity about spiritual things, about supernatural things.
Individualism by its very nature fractures the unity of both spiritual and natural communion. This is why the Church warns us against it in the Compendium.