I want you to observe the Apostle Paul’s choice of words carefully; he said, "And to know the love of Christ which surpasses knowledge." How can you "know" something that "surpasses knowledge?" It sounds superfluous, but it’s a valid statement. There’re two words to mark in that nineteenth verse: "know" and "knowledge." The Greek word translated "know" is "ginosko," while "knowledge" is "gnosis."
"Ginosko" means knowledge and acquaintance, usually by perception or revelation. "Gnosis," on the other hand, refers to knowledge based on science or intellectual activity; abstract knowledge that’s gained from the physical senses. However, the love of Christ is beyond this kind of knowledge; it’s beyond "gnosis."
Putting both together, our opening verse should read, "And to ginosko the love of Christ, which surpasses gnosis." That means we’re to have a revelation of the love of Christ, and not mere intellectual knowledge; we’re to function in the love of Christ beyond our senses and human understanding of love. The natural man (the man that’s not born again), can’t express the love of Christ because he doesn’t have God’s nature of love. This is the reason natural human love, at its best, is fickle and selfish.
However, being born again, you have God’s nature of love; the revelation knowledge of His love is in your spirit. It’s therefore natural for you to express the love of Christ.