A strong belief in free will is an essential component of happiness. We are free to choose good or evil; true happiness consists in choosing what is good. It follows that belief in determinism cannot produce true happiness. It is important that we work, as Catholics and other people of good will, to remind people of the true way to happiness, and to steer people away from thinking that they are helpless with respect to their state of soul.
Modern society and culture does its best to convince us we are nothing but automatons whose actions and condition in life are more or less nothing but the sum of the forces of nature and circumstance. This is a demoralizing, degrading, and depressing force that is mass produced by our society that is without (eschatological) hope.
To counter this culture of demoralization, we must cultivate an ethic of liberty, rightly understood. Liberty is a free gift given to all human persons that enables us to choose happiness despite the circumstances we find ourselves in. This capacity is something real, something concrete we find in our nature. I think our culture, by and large, has lost knowledge of this capacity for greatness – or perhaps we have given it up intentionally, having found its practical difficulties too much to bear. But our willful ignorance does not change the truth. We will only be happy when we choose the good, and this is only possible when we believe such a choice is possible in the first place.