This question arises on the PB. I reply:

Well, as I argue in RRC, there’s a strong case to be made from Scripture and the confession (defined broadly and narrowly) that, in Reformed theology, the public “means of grace” (the “due use of the ordinary means”) are more important than the private. We know precious little about God’s clearly revealed requirements for private piety. What we have are clearly revealed requirements, in the typological revelation about attending to the divinely appointed feasts and other corporate cultic (religious) gatherings. Those were public worship services. In the 18th and 19th centuries, however, the relation between the public and the private became reversed under the influence of pietism.

In confessional Reformed theology, piety, and practice, the private, as important as it be, is secondary to the public. It is through the public reading and preaching of the Gospel that the God has promised to bring his people to faith (Rom 10; HC 65; WSC 88). The holy sacraments are administered in public services. People are disciplined (or ought to be) for failing to attend to the public means of grace. Nevertheless, often times, in my experience (not so much in the last 7-8 years), during elder visits, the first thing they ask about is private, not public piety. When I advocate for two services people sometimes respond by upping the ante by asking, “Why not three services?” There are replies to that question from the RPW and Christian liberty but we could turn that question around regarding private devotions/acts of piety: How often? How long? Who judges? On what basis? I’m much more comfortable disciplining someone for absenting himself from the public means of grace than I am for failing to meet someone’s arbitrary rule of private devotion.

Remember, universal literacy is relatively new. Universal bible ownership is relatively new. That doesn’t mean that people couldn’t have recited passages or even whole books from memory but it means that, for much of world history, God’s people could not have had “devotions” in the way that we think of them.

Private piety and devotion is important. If we neglect private prayer and meditation on Scripture we deprive ourselves on important benefits and blessings. There is probably a correlation between private devotions and maturity but they are not the public means of grace. When it comes to piety, the private flows from the public. The latter is not the joint expression of a hundreds of private religious experiences. Whatever private religious experience we may (or may not) have our Christian life is grounded in the preaching of the Word, especially the gospel, the administration of the sacraments, and public prayers in the context of public worship services.