Reformed and Presbyterian traditions generally hold that the marks of a true church are the pure preaching of the Word, the right administration of the sacraments, and the faithful exercise of church discipline. These marks serve to distinguish the true body of Christ from false sects that may erroneously claim the name.
Across the Reformed tradition, we look to three primary notes to identify the true church. As noted in the Belgic Confession Art. 29, the true church is known by the pure preaching of the gospel, the administration of the sacraments as Christ instituted them, and the exercise of church discipline to punish sin. The Scots Confession Ch. XVIII similarly affirms these, stating that where these notes are seen, there is the true Kirk of Christ, regardless of the number of people present. John Calvin emphasizes in his Institutes 4.1.9 that wherever we see the Word of God sincerely preached and heard, and the sacraments administered according to Christ's institution, we cannot doubt that the church of God exists, for his promise cannot fail.
The ultimate criterion for the true church is its submission to Christ as the only Head. The Second Helvetic Confession Ch. XVII teaches that we do not acknowledge every congregation as a true church simply because it claims the name; rather, we must verify if it hears the voice of the Shepherd. Jesus tells us, "My sheep hear my voice, and I know them, and they follow me" (John 10:27). The true church rests upon the foundation of the prophets and apostles and, while subject to human infirmity and imperfection in its members, it never intentionally deserts the truth of Christ.
While we must be diligent to discern the true church, we must avoid a spirit of rashness. The Westminster Confession Ch. 25 §5 warns that even the purest churches under heaven are subject to mixture and error, and some may degenerate into synagogues of Satan. Yet, we must remember that God uses diversity and even struggle to manifest the truth, as Paul writes: "For there must be also factions among you, that they that are approved may be made manifest among you" (1 Corinthians 11:19). We are called to be vigilant for the integrity of the gospel, yet we trust that God will always preserve his true people, even when they seem few or hidden from our eyes.