I’ll say it briefly: Before you begin showing proof in favor of the Church, you have to compare your own opinions and beliefs with the Word of God and the teachings of the spiritual and right-believing Church Fathers, while over anything that you cannot find such a confirmation for it is more profitable to keep silent. What we think based on our own conclusions is one thing, and the very truth, inviolable, confirmable by the Holy Scriptures, is another.—St. Ambrose of Optina
He [the Lord] commanded us to listen to the teachers and pastors of the Church, appointed and ordained by the apostles… But as for the one who does not listen to it [the Church], let him be to you like a heathen and a publican (Mt. 18:17); and to the apostles He said: he who hears you hears Me, he who rejects you rejects Me (Lk. 10:16). This is how important it is to obey the Holy Church and its dogmas and teachings established by the Ecumenical Councils and the authority of the great Church Fathers. —St. Macarius of Optina
For one’s salvation it is necessary to be a part of the Orthodox Church. At the present time, the number of sects has multiplied… Sometimes sectarians come to me, saying, “We believe in Christ and seek him; where is He?” “He, first of all, especially makes manifest His glorious presence in Heaven, and, secondly, in the Church on Earth. Do you belong to the Church?” “No, we’ve left the Church, but still hope to be saved.” “Well, then your hope is in vain; salvation is impossible outside of the Church.” Those who belong to the Orthodox Church are traveling to the Jerusalem on High, that is, the Kingdom of Heaven, the right way: They sail along the sea of life in a boat, the Helmsman of which is Christ Himself; while those outside the Church attempt to cross the sea on a mere board of wood—which, of course, is impossible—and perish irrevocably. —St. Barsanuphius of Optina
The elder taught his spiritual children to strictly keep and observe the statutes of the Holy Orthodox Church, saying, “If you see, my child, an adherent of a different faith arguing with an Orthodox Christian, desiring to tear him away from the Orthodox Church by means of deceit, then help the Orthodox Christian out. By doing so, you’ll rescue a sheep from the mouth of a lion; but if you remain silent and leave them without help, that would be the same as taking away a redeemed soul from Christ and selling it over to satan”. —St. Anatoly of Optina