It is hard to separate between letters, edicts, and laws. A substantial autocrat, the form of address was much the same, and the force. The extant letters are quite numerous, and those of which we have definite or general mention, many. He seems to have been a most industrious letter-writer. Of the extant letters a majority are undoubtedly or probably genuine. Some, however, need more critical study than seems to have been given to them. 3048 Following is the roughly chronological list, the works being grouped by years. The dating is taken mainly from p. 437 the Migne edition, Ceillier, and Valesius with slight original study. The descriptions are of course from the documents themselves.
1. (313 a.d.) Edict of Constantine and Licinius for the restoration of the Church. In Lact. De M. P. c. 48, and also in Euseb. H. E. 10. 5 (Op. Const. ed. Migne, 105–110). The second edict of toleration. The first edict (Euseb. 8. 17; Lact. De M. P. 34) can hardly be classed among the “writings” of Constantine. This famous second edict grants full religious liberty to the Christians and restoration of their property. Compare section on Acts of Toleration in Wordworths Constantinus.
2. (313.) First letter of Constantine and Licinius to Anulinus. In Euseb. H. E. 10. 5 (Op. Const. ed. Migne, 479–480). Restores goods to the Catholic Christians; written about the same time as the edict of toleration, according to Ceillier.
3. (313.) Second Letter of Constantine to Anulinus. In Euseb. H. E. 10. 7 (Op. Const. 481–2). Ordering that the Catholic clergy be free from public service, that they might not be disturbed in their worship of God.
5. (313.) Letter of Constantine to Melchiades (or Miltiades). In Euseb. H. E. 10. 5 (Op. Const. 477- ). Having received various letters from Anulinus regarding Cæcilian and the Donatists, he summons a council at Rome to consider the matter.
8. (314.) Letter of Constantine to the Bishops after the Council of Arles. In Optat. Mon. vet. p. 287–8 (Op. Const. 487–90). Contains gratulations, reprobations of obstinate schismatists, and exhortations to patience with such obstinateness. It is full of religious expressions, and if genuine, is a most interesting exhibition of Constantines religious position at this time, but it looks suspicious, and probably is not genuine.
9. (314) Letter of Constantine and Licinius to Probianus, the Proconsul of Africa. In Augustine, Ep. 88 (ed. Migne 33  3045), and also in Contr. Cresc. (43  540, also in Op. Const. and tr. Engl. in Schaff, Nicene and Post-Nicene Fathers, 1, p. 370). Orders that the Donatist Ingentius be brought to his court. One text adds Maximianus or Maximus in place of Maximus as epithet of Constantine.
10. (314 or 315.) Letter of Constantine to the Donatist Bishops. In Optat. Mon. vet. p. 290 (Op. Const. ed. Migne  490). As the Donatists were not yet satisfied, he summons them to meet Cæcilian, and promises if they convict him in one particular, it shall be as if in all.
11. (315.) Letter of Constantine to Celsus. In Optat. Mon. vet p. 291 (Op. Const. 489–90). In reply to letter mentioning disturbances of the Donatists, he hints that he expects to go shortly to Africa and settle things summarily.
12. (315.) Fragment of a Letter of Constantine to Eumalius Vicarius. In Augustines Contr. Crest. 3. 71 (ed. Migne 43  541; also Op. Const. 491–2). An extract of six lines, in which he says Cæcilianus was entirely innocent.
13. (316 or 317.) Letter of Constantine to the bishops and people of Africa. Optat. Mon. vet. p. 294 (Op. Const. 491–2). He has tried every way to settle the Donatist disturbances in vain, and now leaves them to God and advises patience.
p. 438 15. (323 a.d.) Law of Constantine respecting piety toward God and the Christian Religion (Ad prov. Pal.). In Euseb. V. C. 2. 24–42; abstr. in Soz. 1. 8 (Op. Const. 253–282). This long edict, addressed to the inhabitants of Palestine, contains an exposition of the prosperity which attends the righteous and the adversity which comes to the wicked, followed by edict for the restitution of confiscated property, the recall of exiles, and various other rectifications of injustices. This is the copy, “or letter,” sent to the heathen population of the empire.
16. (324.) Constantines edict to the people of the eastern provinces concerning the error of polytheism, &c. (Ad. prov. Or). In Euseb. V. C. 48– . This letter, written in Latin and translated by Eusebius, begins with “some general remarks on virtue and vice,” touches on the persecutions and the fate of the persecutors, expresses the wish that all would become Christians, praises God, and exhorts concord.
17. (323 or 324.) Letter of Constantine to Alexander the Bishop and Arius the Presbyter. In Euseb. V. C. 2. 64–72; Gelas. 2. 4; Socr. 1. 7 (Op. Const. 493–502). Expresses his desire for peace, his hope that they might have helped him in the Donatist troubles, his distress at finding that they, too, were in a broil, his opinion that the matters under discussion are of little moment, and what he thinks they are. He exhorts to unanimity, repeats his opinion that the matters are of little moment, mentions his “copious and constant tears,” and finally gets through.
18. (324–5.) Letter to Porphyrius (Optatian). In Migne, Patrol. Lat. 19  393–394 and in various editions of Optatian. This letter to Porphyrius or Optatian was on the occasion of the sending of a poem by the latter for his vicennalia. It expresses his pleasure and his disposition to encourage the cultivation of belles lettres. Compare note on Optatian under sources.
19. (325.) Letter of Constantine the King, summoning the bishops to Nicæa. In Cowper, Syriac Misc., Lond. 1841, p. 5–6. This is translated from a Syriac ms. in the British Museum, written in 501. Gives as reason for the choice of Nicæa the convenience for the European bishops and “the excellent temperature of the air.” This, if genuine, is the letter mentioned by Eusebius, V. C., but it looks suspicious.
20. (325.) Letter of Constantine to the churches after the Council of Nicæa. In Euseb. V. C. 3. 17–20; Socr. 1. 9 (Op. Const. 501–506). Dwells on the harmonious result, especially respecting the Easter controversy, and commends to the bishops to observe what the Council has decreed.
21. (325.) Letter of Constantine to the church of Alexandria. In Socr. 1. 9 (Op. Const. 507–510). Expresses great horror of the blasphemy of Arius, and admiration for the wisdom of the more than three hundred bishops who condemned him.
23. (325.) Letter of Constantine to the churches. In Socr. H. E. 1. 9. A translation of a Syriac translation of this, written in 501, in Cowper, Syriac Misc., Lond. 1861, p. 6–7. Against Arius and the Porphyrians, and threatens that any one who conceals a work of Arius shall be punished with death.
24. (325.) Letter of Constantine to the Nicomedians against Eusebius and Theognis. In Gelas. 3. 2; Theodoret, 1. 20; Soz. 1. 21 (Op. Const. 519–524). A theological discussion partly of the relation of Father and Son, and an attack on Eusebius of Nicomedia.
25. (325.) Letter to Theodotus. In Gelas. 3. 3 (Op. Const. 523–524). Counsels him to take warning by what has happened to Eusebius (of Nicomedia) and Theognis, i.e. banishment, and get rid of such evil influence, if any, as they may have had on him.
p. 439 29. (332 a.d.) Letter of Constantine to the Synod of Tyre deprecating the removal of Eusebius from Cæsarea. In Euseb. V. C. 362; Theodoret, 1. 27 (Op. Const. 543–546).
31. (332.) Second Letter of Constantine to Macarius and the rest of the Bishops in Palestine (to Eusebius). In Euseb. V. C. 3. 52–53 (Op. Const. 539–544). Directs the suppression of idolatrous worship at Mamre.
32. (332.?) Edict against the heretics. In Euseb. V. C. 3. 64–5. Against Novatians, Valentinians, Marcionites, Paulians, Cataphrygians who are forbidden to assemble and whose houses of worship are to be given to the Catholic party.
33. (333.) Letter of Constantine to Sapor, King of the Persians. In Euseb. 4. 9–13; Theodoret, 1. 24 (Op. Const. 545–552). Is mainly a confession of faith commending the Persian Christians to the special care of their king.
34. (333.) Letters of Constantine to Antonius, the monk, and of Antonius to him are mentioned in Athanasius, 1. 855 (Op. Const. 551–552). Constantine and his sons write as to a father. Antony grudgingly replies with some good advice for them to remember the day of judgment, regard Christ as the only emperor, and have a care for justice and the poor.
36. (333.) Letter of Constantine to Eusebius on the preparation of the copies of the Scriptures. In Euseb. V. C. 4. 36; Theod. 1. 15; Socr. 1. 9 (Op. Const. 553–554). Orders fifty copies with directions as to style.
37. (335.) Fragment of the first letter of Constantine to Athanasius. In Athan. Apol.; Socr. 1. 27 (Op. Const. 553–556; Tr. Engl. in Athan. Hist. Tracts, Oxf. 1843, p. 89). The letter summoning to the Council of Tyre, but only a half-dozen lines remain. This bids him admit all who wish to enter the church.
38. (335.) Letter of Constantine to the people of the Alexandrian Church. In Athan. Apol. c. Ar. c. 61 (Op. Const. 559–562; abstract in Soz. 2. 31; Tr. Engl. in Athan. Hist. Tracts, Oxf. 1850, p. 90–92). Is a general lamentation over the dissensions of the Church, with expression of confidence in Athanasius.
41. (335.) Letter of Constantine to Arius. In Socr. 1. 25 (Op. Const. 561–562). Invites Arius to visit him—the famous visit where he presented a confession of faith claimed to be in conformity with that of Nicæa.
42. (335.) A Letter to Dalmatius is mentioned by Athanasius, Apol. 5. 13, but not preserved (Op. Const. 563–564; Tr. Engl. in Athan. Hist. Tracts, Oxf. 1850, p. 94). It required him to make judicial enquiry respecting the charge against Athanasius of the murder of Arsenius.
43. (335.) Celebrated Letter of Constantine concerning the Synod of Tyre. In Euseb. V. C. 3. 42 (Op. Const. 561–564). Exhorts the bishops to give zeal to fulfilling the purpose of the synod in the restitution of peace to the Church.
3. Treaty of peace between Constantine, Sylvester and Tiridates (Op. Const. 579–582). On p. 440 Tiridates compare various sources in Langlois Col. des historiens de…lArmènie, and for literature respecting their authenticity, his note on p. 103.
There are also quite a large number of letters mentioned with more or less description, and a “multitude of letters” (V. C. 3. 24) of which there is no specific knowledge. Of the former may be mentioned that to the inhabitants of Heliopolis, one to Valerius (or Valerianus or Verinus) (Augustine, Ad Donat. p. c. c. 33); one to the Council of Tyre, asking them to hasten to Jerusalem (V. C. 4. 43; Soz. 2. 26); and one acknowledging the copies of the Scriptures prepared at his order, through Eusebius (V. C. 4. 37).
There is of course more or less critical treatment of various letters in critical works on Donatism or Arianism or other special topics. Since writing the above, the exceedingly interesting analysis of sources for early Donatist history, by Seeck, in Briegers Ztschr. f. Kirchenges., 1889, has been examined. He has, like Völter and Deutsch before him, admirable critical studies of certain letters. But a systematic critical study of the Constantinian letters as a whole seem to be still lacking.