3 This also I have been surprised to hear, that certain persons who are in the habit of carrying tales backwards and forwards, and of always adding something fresh to what they have heard, to stir up grievances and disputes between brothers, have succeeded in disquieting you by saying that, when I offer sacrifices to God, I am wont to say this prayer on your behalf: “Grant, O Lord, to John grace to believe aright.” Do not suppose me so untutored as to be capable of saying this so openly.
To tell you the simple truth, my dearest brother, although I continually use this prayer mentally, I have never confided it to the ears of others, lest I should seem to dishonor you. But when I repeat the prayers required by the ritual of the mysteries, then I say on behalf of all and of you as well as others, “Guard him, that he may preach the truth,” or at least this, “Do Thou, O Lord, grant him Your aid, and guard him, that he may preach the word of truth,” as occasion offers itself for the words, and as the turn comes for the particular prayer.
Wherefore I beseech you, dearly beloved, and, casting myself down at your feet, I entreat you to grant to me and to yourself this one prayer, that you would save yourself, as it is written, “from an untoward generation.” Withdraw, dearly beloved, from the heresy of Origen and from all heresies. For I see that all your indignation has been roused against me simply because I have told you that you ought not to eulogize one who is the spiritual father of Arius, and the root and parent of all heresies.
And when I appealed to you not to go astray, and warned you of the consequences, you traversed my words, and reduced me to tears and sadness; and not me only, but many other Catholics who were present. This I take to be the origin of your indignation and of your passion on the present occasion. On this account you threaten to send out letters against me, and to circulate your version of the matter in all directions; and thus, while with a view to defending your heresy you kindle men's passions against me, you break through the charity which I have shown towards you, and act with so little discretion that you make me regret that I have held communion with you, and that I have by so doing upheld the erroneous opinions of Origen.
Source: Letters (New Advent)