What are the signs of the true Church? How can we recognize it? (St. New Martyr Valentine Sventitsky)

In the Holy, true Church, the one in which the fullness of God's grace is revealed, where the holy Sacrament of the Eucharist is celebrated, and in which we can be saved—in this Church, the dogmas professed by the Orthodox Church are professed. In this Church, the canons of the Church established and transmitted to us by the Holy Fathers are observed. In this Church, there exists a consistent, legitimate, and truly ecclesiastical authority. No distortion of dogma is permissible, no arbitrary violation of the canons of the Church is permissible, no arbitrary seizure of ecclesiastical authority is permissible.

If the Holy Church faces any trials, these three foundations must always remain unshakable.
Dogmas must not be distorted for any reason, the statutes must not be denied for any reason, and no illegitimate ecclesiastical authority can be accepted for any reason. Many difficult circumstances can arise in Church life. Terrible times can and have occurred in Church history, when even patriarchs fell away from the true Church. Sins can be forgiven, but falling away from the Church is a sin unto death, a falling away from the very source of salvation.

Thus, one must strengthen one's inner relationship with the Church of Christ. One must be imbued with the firm conviction that only in this true, Orthodox Church can salvation be attained, and each one must strictly guard the purity of the Church of Christ, as if on guard for Orthodoxy, lest they sin here in any way. This requires great patience, great calm, wisdom, and, most importantly, trust in the grace of God.

Source: https://azbyka.ru/otechnik/Valentin_Sventsitskij/monastyr-v-miru/#0_7