The law of his God is in his heart;
his feet do not slip.
When I think about the law of God being in my heart I think of the phrase "I know that by heart." Odd that when we refer to something memorized we say "I know that by heart" and not "I know that by mind." When we really practice something and get good at it, we don't think about it any more. That's why it is so enthralling to watch a master musician perform, or awe-inspiring to watch a world-class athlete at their craft. There is an effortlessness about their mastery.
People don't watch me play guitar, because with the exception of a G and C chord, I cannot play by heart. I play by mind, and my efforts are clumsy and forced. You could tell by watching me that I know what to do, I'm just not very good at doing it. If we could have the law of God in our heart, our lives might look just a little bit smoother. Watching us live might be a joy as we effortlessly navigate the trials of life.