1. Marriage as a Glorious Relationship: Timothy Keller, with insights from his wife Kathy, presents marriage as a glorious relationship that is often misunderstood and mysterious.

2. Instruction for a Successful Marriage: The book offers essential instruction on how to have a successful marriage, providing meaningful insights and wisdom.

3. Biblical Foundation of Marriage: "The Meaning of Marriage" is primarily aimed at Christians and emphasizes the biblical foundation of marriage, making it essential reading for anyone who wants to know God and love more deeply in this life.

4. The Importance of Being Known and Loved: The book delves into the importance of being fully known and truly loved, likening it to being loved by God, and how it liberates, humbles, and fortifies individuals.

5. Challenging Modern-Day Assumptions: Keller challenges modern-day assumptions about marriage and offers a vision of what marriage should be according to the Bible, addressing Christians, skeptics, singles, longtime married couples, and those about to be engage.

6. Insights from the Author's Personal Experience: Timothy Keller draws from his 37-year marriage with Kathy to provide insights and wisdom on the complexities of commitment within marriage.

7. Appeal to a Diverse Audience: The book is designed to appeal to a wide audience, including Christians, skeptics, singles, longtime married couples, and those about to be engaged, offering a vision of marriage rooted in biblical principles.

These key lessons from "The Meaning of Marriage" by Timothy Keller emphasize the book's focus on the biblical foundation of marriage, the importance of understanding and nurturing a successful marriage, and the challenges and rewards of commitment within this sacred relationship.