4 The particular points clearly delivered in the teaching of the apostles are as follow:—
First, That there is one God, who created and arranged all things, and who, when nothing existed, called all things into being— God from the first creation and foundation of the world— the God of all just men, of Adam, Abel, Seth, Enos, Enoch, Noe, Sere, Abraham, Isaac, Jacob, the twelve patriarchs, Moses, and the prophets; and that this God in the last days, as He had announced beforehand by His prophets, sent our Lord Jesus Christ to call in the first place Israel to Himself, and in the second place the Gentiles, after the unfaithfulness of the people of Israel. This just and good God, the Father of our Lord Jesus Christ, Himself gave the law and the prophets, and the Gospels, being also the God of the apostles and of the Old and New Testaments.
Secondly, That Jesus Christ Himself, who came (into the world), was born of the Father before all creatures; that, after He had been the servant of the Father in the creation of all things— “For by Him were all things made” — He in the last times, divesting Himself (of His glory), became a man, and was incarnate although God, and while made a man remained the God which He was; that He assumed a body like to our own, differing in this respect only, that it was born of a virgin and of the Holy Spirit: that this Jesus Christ was truly born, and did truly suffer, and did not endure this death common (to man) in appearance only, but did truly die; that He did truly rise from the dead; and that after His resurrection He conversed with His disciples, and was taken up (into heaven).
Then, Thirdly, the apostles related that the Holy Spirit was associated in honour and dignity with the Father and the Son. But in His case it is not clearly distinguished whether He is to be regarded as born or innate, or also as a Son of God or not: for these are points which have to be inquired into out of sacred Scripture according to the best of our ability, and which demand careful investigation. And that this Spirit inspired each one of the saints, whether prophets or apostles; and that there was not one Spirit in the men of the old dispensation, and another in those who were inspired at the advent of Christ, is most clearly taught throughout the Churches.
Source: De Principiis (New Advent)