This promise is a double-exposure picture of the true follower of God. This person walks rightly (the exterior life) because the law of God is in the heart (the interior life). When the law of God is in the heart, the whole life is right. That is where God intends His law to be, for there it lies, like the tables of stone in the golden ark, in the place intended for it (Deut. 10:2). As Spurgeon said of the law of God: "In the head it puzzles, on the back it burdens, in the heart it upholds."
Notice the choice of words in our text--"the law of his God." When we know the Lord as our God, His law becomes liberty to us. "I will walk at liberty: for I seek thy precepts" (Ps. 119:45). In James 1:25 God's law is called "the perfect law of liberty." Believers are the children of God; therefore the will and word and law of their Father are their delight.
Then, because "the law of God is in his heart, none of his steps shall slide." This is a guarantee that the obedient-hearted believer will be sustained in every step he or she takes, doing what is right, and therefore what is wise. Righteous action is always the most prudent and safe, though it may not seem so at the time. When we keep to the path of God's law, we are walking on the royal highway of God's providence and grace. The law of God, written by the Holy Spirit in the believer's heart, is the new-covenant road map to heaven. "Behold, the days come, saith the Lord, when I will make a new covenant....I will put my laws into their mind, and write them in their hearts" (Heb. 8:8-10).
He who walks righteously walks safely.
MEDITATION PRAYER: "Thou hast enlarged my steps under me, that my feet did not slip" (Ps. 18:36).