Why do we exist? This is one of the greatest philosophical and theological questions man has been attempting to answer for millennia. As Christians, we ask the question within the context of our covenant relationship with God: for what purpose did an all-sufficient God, who needs nothing besides Himself, decide to create us? The Westminster Larger Catechism asks the question this way: "What is the chief and highest end of man?" It answers: "Man’s chief and highest end is to glorify God, and fully to enjoy him forever" (Q&A 1; cf. WSC, Q&A 1). In short, we exist not only to give God glory, as we speak to God in worship through prayer and praise, but also to enjoy Him as He speaks to us in worship through Word and sacrament.
Because Scripture is our ultimate authority, it defines not only our theology but our piety, what we believe about God and how we respond to Him. Piety, then, is our grateful response to what God has done. John Calvin described piety as "that reverence joined with love of God which the knowledge of his benefits induces." The psalmist spoke this way when he said, "Worship the Lord with reverence, and rejoice with trembling" (Ps. 2:11, NASB). The chief and highest way this reverential love is expressed is in public worship.
One aspect that distinguished the Reformed churches from their co-Protestant Lutheran churches was their zeal to engage in the worship of God only on the basis of what the Word of God commanded or implicitly required. To adapt Abraham Lincoln’s "Gettysburg Address," the Reformers believed that worship is of God, by God, and for God. For the Reformed, this meant that all unbiblical ceremonies were abolished for public worship. In fact, Calvin was so adamant about this point that he said the entire project of the Protestant Reformation was about worshiping God in a way that was pleasing to Him. This point even led the great English matriarch, Queen Elizabeth I (1533–1603), to describe the Reformed churches on the continent of Europe as "more reformed" than the Lutheran churches.
The Belgic Confession of Faith links the Reformed churches' belief in the sufficiency of the Word of God to the area of worship when it says, "For since the whole manner of worship which God requires of us is written in them at large, it is unlawful for any one, though an Apostle, to teach otherwise than we are now taught in the Holy Scriptures: nay, though it were an angel from heaven, as the Apostle Paul saith" (Art. 7). “The whole manner of worship which God requires” is found in the Scriptures. This means we come to worship on God’s terms, not ours; that we do in worship what God wants, not what we want.
Continuing in a later section, the Belgic Confession says:
we reject all human inventions, and all laws which man would introduce into the worship of God, thereby to bind and compel the conscience in any manner whatever. Therefore we admit only of that which tends to nourish and preserve concord and unity, and to keep all men in obedience to God. (Art. 32)
The Word, then, contains all we need in order to know how to worship; therefore, we reject all human-made laws or elements of worship. This is most memorably and succinctly stated in the Heidelberg Catechism, which says:
What does God require in the second commandment?
That we in no wise make any image of God, nor worship him in any other way than he has commanded us in his Word. (Q&A 96)
Over the centuries, Reformed churches came to call these ideas the "Regulative Principle of Worship." The Regulative Principle of Worship holds that we worship God in the manner He has commanded us in His Word. As the Westminster Confession says,
But the acceptable way of worshipping the true God is instituted by himself, and so limited to his own revealed will, that he may not be worshipped according to the imaginations and devices of men, or the suggestions of Satan, under any visible representations or any other way not prescribed in the Holy Scripture. (21.1)
In the Reformed churches, we hold to this principle because we take the Bible seriously. It is God’s Word to us for our faith, as well as for our worship and Christian life. Scripture alone is our ultimate rule, and it sufficiently gives us "all things that pertain to life and godliness" (2 Peter 1:3). So it alone governs the substance of what we do in worship.