Ignatius of Antioch is among the earliest church fathers, those who served the church in the generation following the passing of the apostles. His writings, at least inasmuch as they have come down to us faithfully preserved, give us insight into the belief and practice of the early church.
We have many letters ascribed to Ignatius of Antioch. Some are nearly universally considered to be fraudulent; written at much later dates to dishonestly enlist the honorable church father and martyr as a supporter of this or that theological position in controversies that arose later in church history. Others are probably authentic, although several different versions of those letter have come down to us. For these letters, there are shorter and longer versions coming form different manuscript traditions. The longer versions likely include extra material added in following centuries.
Reaction to these textual issues varies, with some wanting to ignore the letters altogether as unreliable, while others strongly believe the reliability of the letters and see them as valuable sources of historical theology and practice.
It is not my intention to weigh in on the textual issues surrounding these letters, or their reliability or lack thereof. Perhaps they really represent Ignatius’s beliefs accurately; perhaps they would better be said to represent the teachings of a dishonest third or fourth century tamperer. Regardless, these letters are rich with attestations to the truth that the one God of the Christian faith is the person of the Father, as well as other important elements of trinitarian doctrine. For our purposes, and with the above disclaimers, I will treat these as representative of the true beliefs of Ignatius. Whoever the author may be, these letters serve as a helpful witness to the classical trinitarianism of the apostolic era:
“On this account also they were persecuted, being inspired by His grace to fully convince the unbelieving that there is one God, who has manifested Himself by Jesus Christ His Son, who is His eternal Word, not proceeding forth from silence, and who in all things pleased Him that sent Him.” Epistle to the Magnesians (shorter version), Chapter VIII.
“If any one confesses Christ Jesus the Lord, but denies the God of the law and of the prophets, saying that the Father of Christ is not the Maker of heaven and earth, he has not continued in the truth any more than his father the devil, and is a disciple of Simon Magus, not of the Holy Spirit. If any one says there is one God, and also confesses Christ Jesus, but thinks the Lord to be a mere man, and not the only-begotten God, and Wisdom, and the Word of God, and deems Him to consist merely of a soul and body, such an one is a serpent, that preaches deceit and error for the destruction of men.” Epistle to the Philadelphians (longer version), Chapter VI.
“The prophets also, when they speak as in the person of God, [saying,] “I am God, the first [of beings], and I am also the last, and besides Me there is no God,” concerning the Father of the universe, do also speak of our Lord Jesus Christ.” Epistle to the Antiochians, Chapter III.
“The Evangelists, too, when they declared that the one Father was “the only true God,” did not omit what concerned our Lord, but wrote: “In the beginning was the Word, and the Word was with God, and the Word was God.” Epistle to the Antiochians, Chapter IV.
“For if there is one God of the universe, the Father of Christ, “of whom are all things;” and one Lord Jesus Christ, our [Lord], “by whom are all things;” and also one Holy Spirit, who wrought in Moses, and in the prophets and apostles; and also one baptism, which is administered that we should have fellowship with the death of the Lord; and also one elect Church; there ought likewise to be but one faith in respect to Christ. For “there is one Lord, one faith, one baptism; one God and Father of all, who is through all, and in all.”” Epistle to the Philippians, Chapter I.
“There is then one God and Father, and not two or three; One who is; and there is no other besides Him, the only true [God]. For “the Lord thy God,” saith [the Scripture], “is one Lord.” And again, “Hath not one God created us? Have we not all one Father? And there is also one Son, God the Word. For “the only-begotten Son,” saith [the Scripture], “who is in the bosom of the Father.” And again, “One Lord Jesus Christ.” And in another place, “What is His name, or what His Son’s name, that we may know?” And there is also one Paraclete. For “there is also,” saith [the Scripture], “one Spirit,” since “we have been called in one hope of our calling.”” Epistle to the Philippians, Chapter II.
“Ignatius answered, “Thou art in error when thou callest the dæmons of the nations gods. For there is but one God, who made heaven, and earth, and the sea, and all that are in them; and one Jesus Christ, the only-begotten Son of God, whose kingdom may I enjoy.”” Ignatius before Trajan, at his martyrdom. From the Martyrdom of Ignatius, Chapter II.
The agreement of these statements with later church fathers like Irenaeus can be easily recognised (see: https://contramodalism.com/2017/03/08/i-believe-in-one-god-the-father-almighty/ ). The identicality of what he teaches on this point with what scripture teaches is clear enough from the quotes themselves.