The theologian A.A. van Ruler often used three words to describe the church's confessions: a staff, a stick, and a song. Using these three images, van Ruler summarized the various functions of the confessions for the life of the church--leading and guiding her, correcting and defending her, and leading her in praise and worship of the Triune God.
I'd like to borrow these three images to unfold the meaning of the marks of the church as articulated in the Belgic Confession. The true church, as Belgic Confession Article 29 states, can be recognized if:
The church engages in the pure preaching of the gospel; it makes use of the pure administration of the sacraments as Christ instituted them; it practices church discipline for correcting faults..By these marks one can be assured of recognizing the true church--and no one ought to be separated from it.
We find in this definition three marks to correspond to the three images given by van Ruler: the staff of the pure preaching of the gospel; the stick of church discipline; and the song of the sacraments.
A staff is an object that guides, and that which serves as the guide for, and foundation of, the church and its role in God's mission is the gospel of Christ and the kingdom. The question is: What is the pure preaching of this gospel that guides the true church? At the heart of the pure preaching of the gospel is the doctrine of justification by grace alone through faith alone in Christ alone. However, this is only the narrow lens of the good news found in Christ and the kingdom. The wider lens of this good news is that God's plan to redeem and restore all creation from the sinful rebellion of humankind and its effects has powerfully broken into the world in Christ.
Practically, this mark is significant as a guide in that, in the midst of many ideas and various issues that seek to pull the church's focus elsewhere, it keep the church centered on and rooted in what is most important. The church, and the Christian religion, can be about many things, even good things, but apart from this mark, it will fail to live into God's mission and its calling to witness to Christ and the kingdom. Additionally, the inclusion of the phrase "pure preaching" is a reminder that simply having an orthodox understanding of the gospel means nothing if it is not, in fact, proclaimed.
A stick is an apt symbol for church discipline. A stick can be used for defense, to point something out, or to bring correction. Church discipline does all three. First, church discipline defends the truth, as well as the Lord's reputation, and his desire for holiness in the church. Second, it keeps the church, and in particular its teachers and preachers, accountable by pointing out the true doctrine of God's Word in the midst of many false teachings that seek to sneak in. Third, church discipline can do just that--discipline, or correct those who willfully sin or reject the sound doctrine of the church. Central to this use is that it is always intended to be restorative, with the return of the wandering sheep or the prodigal son as the end goal.
Contemporarily, it seems appropriate to emphasize church discipline, as the practice seems to have gone out of vogue--whether from its misuse, abuse, or the generally negative perception of the word in today's culture--and numerous teachings that contradict the Word and the confession have led people astray, causing division in the church. In the end, the church needs discipline, for the glory of God, the good of the church, and its gospel witness in the world.
This leads to the third mark: the pure administration of the sacraments. The confession identifies two sacraments--baptism and the Lord's Supper--as instituted by Christ in Scripture. As with discipline and the preaching of the gospel, the focus of the sacraments is on the plain and authoritative teaching of God's Word.
The sacraments serve as a song in that they declare and celebrate the work of God in the gospel, and praise his name for his promises, provision, and presence given to us in such visible ways. There is a doxological character to the sacraments, as we receive them from God and, in response, worship him for these gifts of grace that nourish and sustain our faith. Yet, the pure administration of the sacraments are not simply about praise, but also the means of grace necessary for those who are a part of the church to faithfully live as citizens and ambassadors of the kingdom.
These three marks--the pure preaching of the gospel, the practice of church discipline, and the pure administration of the sacraments--are doctrinal products of their time, but also very practical for our day. They are a staff, a stick, and a song for the church as she seeks to fulfill the mission given her by Christ in the unfolding of God's redemptive plan; for his glory and the good of God's people.
 Allan J. Janssen, Confessing the Faith Today: A Fresh Look at the Belgic Confession (Eugene, OR: Wipf and Stock, 2016), 5-6.
 Michael Goheen, Introducing Christian Mission Today (Downers Grove, IL: IVP Academic, 2014), 39.
 Daniel Hyde, With Heart and Mouth: An Exposition of the Belgic Confession (Grandville, MI: Reformed Fellowship, Inc., 2008), 398.