Book II. He begins with the command to baptize all nations as a summary of the faith; this by itself would suffice were not explanations rendered necessary by heretical misrepresentations of its meaning. For (§§ 3, 4) heresy is the result of Scripture misunderstood; and here we must notice that Scripture is regarded as ground common to both sides. All accept it as literally true, and combine its texts as will best serve their own purposes. Hilary, regarding all heresies as one combined opposition to the truth, makes the two objections that their arguments are mutually destructive, and that they are modern.
Then in § 5 he expresses the awe with which he approaches the subject. The language which he must use is utterly inadequate, and yet he is compelled to use it. In §§ 6, 7 he begins with the notion of God as Father; in §§ 8-11 he proceeds to that of God the Son. He states the faith as it must be believed; it is not enough (§§ 12, 13) to accept the truth of Christ's miracles. The mystery, as it is revealed in St. John i. 1-4, must be the object of faith. In §§ 14-21 he expounds this passage in the face of current objections, and then triumphantly asserts that all the efforts of heresy are vain (§ 22).
He advances proof-texts in § 23 against each objector, and then points out in §§ 24, 25 our indebtedness to the infinite Divine condescension thus revealed. For, in all the humiliation to which Christ stooped the Divine Majesty was still inseparably His, and was manifested both in the circumstances of His birth and in His life on earth (§§ 26-28). The book concludes (§§ 29-35) with a statement of the doctrine of the Holy Ghost, as perfect as in the undeveloped state of that doctrine was possible.
Source: On the Trinity (New Advent)