Gaudium et Spes 19§1 says this about why man exists:
For man would not exist were he not created by God’s love and constantly preserved by it; and he cannot live fully according to truth unless he freely acknowledges that love and devotes himself to his Creator.
[cf. also CCC §27]
That might sound a bit esoteric or head-in-the-clouds at first: “live fully according to truth.” Let’s consider it a moment.
We say a thing is true when it corresponds to reality: “The sky is blue” is true because the sky really is blue; “It’s raining” is false (at least for me, at this moment) because it is not raining where I am. The first affirmation is true because it corresponds to the way things actually are. The second is false because it doesn’t.
When the Fathers of Vatican II say that a man “cannot live fully according to truth unless he freely acknowledges [God’s] love and devotes himself to his Creator,” one thing that is implied is that the man who doesn’t do these things is in part living according to falsehood. His life is, to that extent, ordered according to error.
What happens when a man orders his life according to a mistake? Well, he might make bigger mistakes as a result. We considered this terrible consequence in another post. Bad things may happen when we make mistakes like this, because we’re thinking and acting as though the world is one way when it’s actually something different.