I never get tired of reading C.S. Lewis. Mere Christianity is quite simply an amazing book. I peer into it every now and again and come away in awe of Lewis' penetrating use of analogy.
In fact, that is just why a vague religion--all about feeling God in nature, and so on--is so attractive. It is all thrills and no work: like watching the waves from the beach. But you will not get to Newfoundland by studying the Atlantic that way, and you will not get eternal life by simply feeling the presence of God in flowers or music. Neither will you get anywhere by looking at maps without going to sea. Nor will you be very safe if you go to sea without a map. (pp. 154-155).
As is often the case, "both/and" describes the course we should take. Experience of God (going to sea) and learning about God (looking at maps). Lewis reminds us about an important aspect of faith. The Catechism of the Catholic Church defines the theological virtue of faith as:
Faith is the theological virtue by which we believe in God and believe all that he has said and revealed to us, and that Holy Church proposes for our belief, because he is truth itself. (Section 1814).
The definition does not stop at "Faith is the theological virtue by which we believe in God." The reason is that Satan believes in God, but he does not believe all that God has said and revealed and that the Church proposes for our belief. That is why simply experiencing God is not enough. "It is all thrills and no work." While we certainly cannot work our way to heaven (that is the heresy of Pelagianism that was condemned in the early Church), it is also true that we cannot simply say "Yes, I believe" and not have that belief transform our lives into ones where we follow God's will and do good things.
Besides faith, we also need the theological virtue of charity. The Catechism defines charity as:
Charity is the theological virtue by which we love God above all things for his own sake, and our neighbor as ourselves for love of God. (Section 1822).
We cannot simply say, "I love you, Lord" and not have obligations, any more we can tell our wife or husband or children "I love you" and not have obligations to them. Men who abuse their wives often apologize for abusing them and then tell them how much they love them, but their actions say otherwise. Love is not a feeling, but an act of the will.
Vague religion is very popular these days. That is because it is easy. True religion, like true love, makes demands on us.