Catholic philosopher extraordinaire Peter Kreeft posits the following 12 lessons as the most important ones he ever learned:
1. There Is Only “One Thing Necessary.”
2. The Way to Happiness Is Self-forgetful Love
3. “In Everything God Works for Good with Those Who Love Him.”
4. Everything Is a Gift from God
5. God Did It All for Me Alone
6. We Will Be Perfectly and Uniquely Fulfilled by God’s Love in Heaven
7. The Gift of Love Is Ours for the Taking
8. Because of the Love of God, We Can Love Our Neighbor Freely
9. God’s Love Is an Objective Reality that Makes a Real Difference
10. We Were Made to Be United to God Forever
11. The Desire for Joy Points Us to the Love of God
12. Romantic Love Reveals the Beloved and Is Meant to Point Us toward Union with God
Read the details of each of these ideas here.