The work with which we are especially concerned at this time is the Church History, the original Greek of which is still extant in numerous mss. It consists of ten books, to which is added in most of the mss. the shorter form of the Martyrs of Palestine (see above, p.29). The date of the work can be determined with considerable exactness. It closes with a eulogy of Constantine and his son Crispus; and since the latter was put to death by his father in the summer of 326, the History must have been completed before that time. On the other hand, in the same chapter Eusebius refers to the defeat of Licinius, which took place in the year 323 a.d. This gives a fixed terminus a quo. It is not quite certain from Eusebius' words whether the death of Licinius had already taken place at the time he wrote, but it seems probable that it had, and if so, the completion of the work must be put as late as the summer of 324. On the other hand, not the slightest reference is made to the Council of Nicæa, which met in the summer of 325; and still further the tenth book is dedicated to Paulinus, at one time bishop of Tyre and afterward bishop of Antioch (see Euseb. Contra Marc. I.4, and Philost. H. E. III.15), who was already dead in the summer of 325: for at the Nicene Council, Zeno appears as bishop of Tyre, and Eustathius as bishop of Antioch (see for further particulars Lightfoot, p.322). We are thus led to place the completion of the History in the year 324, or, to give the widest possible limits, between the latter part of 323 and the early part of 325 a.d.
But the question has been raised whether the earlier books may not have been composed some years before this. Lightfoot (following Westcott) supposes that the first nine books were completed not long after the edict of Milan and before the outbreak of the quarrel between Constantine and Licinius in 314. There is considerable to be said in favor of this theory. The language used in the dedication of the tenth book seems to imply that the nine books had been completed some time before, and that the tenth is added as a sort of postscript. The close of the ninth book strengthens that conclusion. Moreover, it would seem from the last sentences of that book that Constantine and Licinius were in perfect harmony at the time it was written, a state of affairs which did not exist after 314. On the other hand, it must be noticed that in Book IX. chap.9 Licinius' |madness| is twice referred to as having |not yet| seized him (in §1 oupo manentos tote, and in §12 oupo tote eph' hen husteron ekpeptoke manian, ten di?noian ektrapeis). It is necessary either to interpret both these clauses as later insertions (possibly by Eusebius' own hand at the time when he added the tenth book; cf. also p.30, above), or to throw the composition of the ninth book down to the year 319 or later. It is difficult to decide between these alternatives, but I am inclined on the whole to think that Westcott's theory is probably correct, and that the two clauses can best be interpreted as later insertions. The very nature of his History would at any rate lead us to think that Eusebius spent some years in the composition of it, and that the earlier books, if not published, were at least completed long before the issue of the ten books as a whole. The Chronicle is referred to as already written in I.1; the Eclogæ Proph. (? see below, p.85) in I.2 and 6; the Collection of Ancient Martyrdoms in IV.15, V. preface, 4, and 22; the Defense of Origen in VI.23, 33, and 36; the Life of Pamphilus in VI.32, VII.32, and VIII.13. In VIII.13 Eusebius speaks also of his intention of relating the sufferings of the martyrs in another work (but see above, p.30).