Letter 10042: If, most dear brother, we consider attentively how great is the excellence of peace, we shall recognize with what earnestness it should be cultivated by us. For indeed our Lord and Redeemer vouchsafed to leave and give it as a great boon to His disciples, that He might thereby make those who were united to Him in firmness of faith His associates...

Gregory to Eusebius, Archbishop of Thessalonica. Most dear brother, if we truly consider how excellent peace is, we will recognize how earnestly we must cultivate it. Our Lord and Redeemer left peace as a great gift to his disciples, making those who held firm in faith his companions through love. As it is written, "Blessed are the peacemakers, for they shall be called children of God" (Matthew 5:9). Whoever desires to be the Father's heir must accept being his child by keeping the peace. Whoever gives place to discord cuts himself off from so great a gift. Since, by God's mercy, the purity of your faith has been declared to us with Catholic integrity, I am deeply surprised that you would allow people whom you know to believe rightly to be needlessly scandalized by the faults of certain others -- so that your own Fraternity's reputation is clouded by their guilt. How can someone avoid suspicion of error who tolerates those who are in error? What can he expect people to think of him, if he does not take care to clear away through open correction what the fervor of faith demands be corrected? Here is the specific problem: I am told that your presbyter Luke and one named Peter refuse to accept the Council of Chalcedon [the ecumenical council of 451 that defined Christ as one person in two natures], and that this has caused no small offense among your orthodox faithful. Their zeal is not only praiseworthy but deserves to be encouraged. I urge your Fraternity: investigate this without delay, with full energy and care. If those persons are found innocent of that error, reassure your people and remove the scandal. Among all heresies, have them specifically anathematize Severus and Nestorius [leaders of opposing christological heresies -- Monophysitism and Nestorianism]. If they are found guilty, proceed with the canonical penalties.

Book X, Letter 42 To Eusebius, Archbishop of Thessalonica. Gregory to Eusebius, etc. If, most dear brother, we consider attentively how great is the excellence of peace, we shall recognize with what earnestness it should be cultivated by us. For indeed our Lord and Redeemer vouchsafed to leave and give it as a great boon to His disciples, that He might thereby make those who were united to Him in firmness of faith His associates in loving participation with Himself. For it is written, Blessed are the peacemakers, for they shall be called the children of God Matthew 5:9. Whosoever, then, desires to be the father's heir, let him, by keeping peace, not refuse to be his child. For he who gives place to discord surely makes himself to be without lot in so great a gift. Seeing then that by the mercy of God the purity of your faith has been declared to us, as was meet, with Catholic rectitude, we are taken up with great surprise that you should suffer those whom you know to believe well and to think aright to be needlessly scandalised by the fault of certain persons, so that the reputation of your Fraternity is clouded by the guilt of others. For how can one avoid suspicion of error who extends sufferance to them that are in error? Or what estimate of himself can he expect, if he provides not for purging by open satisfaction what fervour of faith requires to be purged? For indeed it is said that Luke your presbyter and Peter refuse to receive the Chalcedonian synod, and that on this account the hearts of your orthodox children are perturbed with no slight offense . And, since their zeal is not only to be praised but also to be altogether cherished, we exhort that the care of your Fraternity hesitate not to investigate the matter with all activity and solicitude. And, if those persons should be found innocent of that pravity, remove offense from the minds of your children by giving them satisfaction, and among all heresies anathematise especially Severus and Nestorius, so that purification may engender charity among those with whom a sinister suspicion concerning those heretics has, out of love of the faith, produced dissension; and that one feeling of concord may salubriously knit together those whom a pure and single confession of Catholic truth unites. Nor let the doubters be thought unworthy of satisfaction, since we are instructed by the Divine voice, Despise not one of these who are the least Matthew 28:18. Whoever, then, desires not that he who instructs us should be despised, let him not reject the words of the instructor; since he also of whom our Redeemer testified that he was a vessel of election unto Himself admonishes us to keep the unity of the Spirit in the bond of peace Ephesians 4. Hence whosoever refuses not to be held by this bond of salvation, let him study the things that make for peace, and afford no place for the foe; so that, having been enabled to advance by the fierce dissension of brethren, he may be more stoutly trampled on, when unity is established. If however, as we do not expect, they should be found to be wounded by the dart of this error, the cure of ecclesiastical exhortation must be applied to them, so that they may either remain among the Lord's sheep if healed, or be cut off from the unity of the ecclesiastical body; to the end that from a slight loss there may be a great gain, and that the removal of a part may make the whole body free. For it is the care also of a provident shepherd not to delay casting out from consort with his sound sheep a sickly one that admits not of cure, lest it should contaminate others with the taint of its sickness, knowing that he cannot preserve the soundness of the rest but by the ejection of this one. Accordingly I once more warn you in brotherly charity to investigate this matter with the utmost vigilance, and to observe what we have written with the utmost care, lest by consort with others you should make the right faith which you hold doubtful. For he who does not correct things that should be cut off commits them. Wherefore you must take thought with great solicitude and with great provision in all ways, that the persons of those men be not an offense to others, or common opinion injurious to you; that so a shepherd's gains may accrue to your Fraternity from the sheep committed to you all the more as both sincere love and approved care shall have made you solicitous for their custody.