The first commandment, which calls us to have no other gods but the one true God and Creator of all (Deut. 5:7), is the fundamental law of the Decalogue—the Ten Commandments. Every other regulation is grounded in shunning all forms of idolatry in favor of singular devotion to the triune Lord of the universe. We obey this God and refrain from adultery, murder, theft, and everything else He forbids because He is our transcendent Creator and has the absolute right to impose obligations upon us.
In expounding the first commandment, the Heidelberg Catechism tells us that having no other god means, in the first instance, that we refuse "all idolatry, sorcery, superstitious rites, and prayer to saints or to other creatures" (A. 94). None of these things is transcendent, so none of these things deserves our ultimate allegiance. They cannot save in the day of trouble, as Scripture tells us again and again (Judg. 2:11–16; 2 Kings 18:13–19:36; Jer. 11:12). God stands alone in His holiness, majesty, and power (Ex. 15:11). To turn from the one true God and trust in anything else is to endanger our salvation (Gen. 19:26; John 14:6; Heb. 3:12–14).
When he looks at the first commandment, John Calvin agrees that God "enjoins us to put far from us all impiety and superstition, which either diminish or obscure the glory of his divinity." However, "for the same reason he commands us to worship and adore him with true and zealous godliness" (Institutes 2.8.16). To have no other god is to be fully and unashamedly committed to the Creator and Sovereign of the universe. There is no neutral ground—it is not enough to ignore idols and be indifferent to the Lord God Almighty, for indifferent people trust in themselves and so commit idolatry. Instead, we must serve the Lord with all of our hearts and with all of our souls, as today’s passage indicates (Deut. 10:12). As question and answer 94 of the Heidelberg Catechism put it, we must "love, fear, and honor God" wholeheartedly.
Of course, we cannot love, fear, or honor someone truly if we know nothing about that person. The same thing applies to the Lord. We must diligently study His Word and seek Him in prayer so that we might know Him and, knowing Him, that we might love Him (Ps. 9:10; Prov. 2:1–5).
The Westminster Shorter Catechism tells us that man's chief end is to glorify God and enjoy Him forever. Yet to glorify and enjoy Him, we must know Him as He is in His holiness, sovereignty, and love. We must understand what He has done for His people and His promises to bless those who follow His ways. The only way to do this is to study His revelation and sit under sound preaching that explores and applies the Word of God to God's people.