Reformed theology has always maintained that there are three specific marks of a true church of Jesus Christ: the true preaching of the Word of God, the proper administration of the sacraments, and the faithful exercise of church discipline (a fourth mark, church government, is sometimes listed). Louis Berkhof, Manual of Christian Doctrine, pp. 285-287. The latter mark, church discipline, is one of the ways by which "the church will...be purified." The WCF XXXIV, 4. 
The former two marks, along with prayer, which has been studied in chapter three, are also considered to be "means of grace," by which Christians grow in their relationship with the Triune God. Scriptural support for this terminology - "means of grace" - can be found in 2 Peter 3:18, where the apostle enjoins Christians to "grow by means of grace." The Greek en chariti, used here by Peter, is an instrumental dative. As stated in the Shorter Catechism (Q 88), "The outward and ordinary means whereby Christ communicateth to us the benefits of redemption, are His ordinances, especially the Word, sacraments, and prayer; all which are made effectual to the elect for salvation."

1993 Issue 9 - He Shall Glorify Me: Doctrine of the Holy Spirit in the Westminster Standards Part 6 - Counsel of Chalcedon PDF

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