But the word is very near you, in your mouth and in your heart, that you may observe it.
This statement comes on the tail end of Moses telling Israel that God’s Law is not unobtainable, but rather is in their possession. He contrasts the distances to which some might say they would have to go to get God’s Law with just how close that very Law is.
C.S. Lewis, in Mere Christianity, takes a position that there is a fundamental Law within every human being. There are some things which human beings generally agree are not good. This is an example of how God’s Law is in your heart.
But there is a more direct application for me, as a person who has read The Bible for years (nearing three decades, at this point). Having read God’s Word for so long, there is a great deal of it squirreled away in secret corners of my heart and mind and those secreted nuggets often find their way to the surface when I need them. Is a decision right or wrong for me? To the surface rise passages I did not take pains to memorize, but they are there just the same. The Word is in my heart, tucked away by years of reading. More, though, I grew up in a time when The Bible was liberally quoted in literature, theater, television, and cinema. Allusions were made to it. Proverbs from it were quoted as answers to life questions. The Bible was in the mouth of everyone around me. This has changed for the culture around me, but has not changed for me. I often find myself answering difficult questions with scripture. The Word is in my mouth, situated by habit and the knowledge that even a fool like me sounds wise when speaking the Word of God plainly.
God’s Word is, in fact, in my heart and in my mouth, but the last component is the kicker: observe it. It is not enough that God’s Word be in my mouth and in my heart, I must obey it. I must do what God has told me to do. James will (much) later write that the one who knows the good he ought to do and does not do it sins. If God’s Word is in my mouth and heart and I’m not doing the things I know to do, then I am sinning.