St. Bernard of Clairvaux, a Cistercian monk in 12th century France, rose from relative obscurity to influence many aspects of church life during his time. Born within a wealthy, Bernard forsook all of that to bring enter into a small monastic community in Cîteaux, influencing his five brothers and around twenty-five friends to join with him. Over time, his strict renunciation of life’s pleasures and influential love of God brought him to leadership, first in forming a new monastic community in Clairvaux and later to be an advisor to church leaders. His least admirable legacy was helping to whip up interest in the Second Crusade.
However, what Bernard is often best-known for today is his writings on the love of God. His work, On Loving God (available in full here or summarized here), provides one of the most powerful explanations of both God’s love for human beings and human love returning toward God. Most notably, he outlines four degrees of love for God, which have provided a framework for growing in love toward God for many over the years. In fact, I first heard about Bernard of Clairvaux in a seminar on the love of God that I attended during my college years while at the Urbana conference. The speaker referenced Bernard again and again, and I figured this was someone who I needed to know more about.
When I returned to school after Christmas break at Wheaton College, I scoured the lower level of Buswell Memorial Library until I found works by Bernard of Clairvaux. This led me to a four-volume set of his 86 sermons on the Song of Songs (excerpts available online here). Convinced that, as Paul writes in Ephesians 5, the relationship of a husband and wife in Christ mirrors the love relationship that exists between Christ and the Church, Bernard preached these sermons on the Song of Songs as a means to better understand God’s love in Christ for His people. When you read those sermons, you know that Bernard knew the love of God that surpasses all our knowing. Eugene Peterson, that rugged pastor to pastors, once wrote: “Love is Bernard’s theme, a non-sentimental, hardheaded and warmhearted love that is equally informed by self-knowledge and God-knowledge” (Take and Read 10).
Reflecting on God’s love and our love back to God, Bernard once wrote to a friend:
You wish me to tell you why and how God should be loved. My answer is that God himself is the reason why he is to be loved. As for how he is to be loved, there is to be no limit to that love. 
If you are looking for a good guide into the love of God, I cannot recommend too many more heartily than St. Bernard of Clairvaux.
 Bernard of Clairvaux, “On Loving God,” https://www.christianitytoday.com/history/issues/issue-24/on-loving-god.html.