…I pray Thee, O my Lord, have mercy on my soul, that it may not encounter the malice of Thy enemies, but that Thy angels may receive it and lead it through the trials of the darkness after death towards the light of Thy mercy. (A Life of St. Theodosius, The Way of the Pilgrim and Other Classics of Russian Spirituality edited by G.P. Fedotov [kindle version])
Lord, give the blessing!
I have something to say to you, God-loving Prince! I, Theodosius, the wretched slave of the All-Holy Trinity: the Father, the Son and the Holy Spirit, was born and raised in the pure and Orthodox faith and in good Orthodox teaching by my father and mother.
Beware, my child, of those who are crooked in faith, and beware all their conversations, for our land is full of them.
If a man will save his soul, this is possible only by living in the Orthodox faith. For there is no other faith better than our pure, holy, Orthodox faith. Living in this faith, not only will you be delivered from sins and eternal torments, but you will also become a partaker of eternal life, and you will rejoice endlessly with the saints. But those who live in another faith will not see eternal life.
Also, my child, it is not good to praise a foreign faith. He who praises a foreign faith is doing the same as if he were blaspheming against his own faith. But if someone will praise both his own faith and a foreign faith, then he is two-faithed, and is close to heresy.
And so, my child, beware of them and always stand for your own faith. Do not be friendly with them, but flee from them and struggle in your own faith through good works. Give alms not only to those of your own faith, but also to those of other faiths. If you see someone naked or hungry, or who has fallen into misfortune, – whether he is a Jew, or a Turk, or a Latin, – be merciful to everyone, deliver him from his misfortune, as far as you are able, – and you will not be deprived of a reward from God. For God Himself in this age pours out His mercies not only on Christians, but also on the unbelievers. God cares for pagans and unbelievers in this age, but in the future age they will be strangers to the eternal good things. But we who live in the Orthodox faith will both receive all good things here and will be saved in the future age by our Lord Jesus Christ.
My child! If it will be necessary for you even to die for this holy faith, go with boldness to death. In this way did the saints die for the faith, and they now live in Christ.
My child, if you see those of other faiths quarrelling with the Orthodox and trying to seduce them away from the Orthodox Church, help the Orthodox. In this way you will deliver the sheep from the mouth of the lion. But if you are silent and leave him helpless, that is the same as if you took away a soul that had been redeemed by Christ and handed him over to Satan.
If someone says to you: “Both your and our faith are from God”, you, my child, must reply to him as follows: “O crooked in faith! Or do you think that God, too, is two-faithed? Have you not heard what the Scripture says: ‘There is one God, one faith, one baptism’ (Ephesians 4.5)? How many years have you kept the right, Apostolic faith, and now through Satan’s insinuation you have been corrupted into evil belief. Have you not heard the teaching of the Apostle Paul: ‘Even if an angel should come from heaven and preach to you a Gospel that we have not preached to you, let him be anathema’ (Galatians 1.8)? But you have rejected the apostolic teaching and the decrees of the Holy Fathers and accepted a wrong and corrupted faith, filled with all destruction. Therefore you are rejected by us. You are dead, and the sacrifice you offer [i.e., the Eucharist] is dead. But we offer a living sacrifice to the living God, a pure and undefiled sacrifice, so as to obtain eternal life in Christ Jesus our Lord. To Him be glory. Amen. (Testament of St. Theodosius of the Kiev Caves to the Great-Prince Iziaslav on the Orthodox Christian Faith)