Irenaeus of Lyons, unlike many church fathers, is one relatively well known to modern Christians. The second century bishop of Lyons is famous for his staunch opposition to the various pseudo-gnostic heresies that faced the church in his day, and especially for the multi-volume work Against Heresies that he authored to combat them. He also authored a lesser-known work summarizing an orthodox understanding of the Christian faith and proving its tenets from the scriptures called Demonstration of the Apostolic Preaching, which I highly recommend.
Here I want to briefly examine some quotes from his writings that highlight his strongly held and clearly-articulated belief that the one God of the Christian faith is the person of the Father in particular.
“And others of them, with great craftiness, adapted such parts of Scripture to their own figments, lead away captive from the truth those who do not retain a stedfast faith in one God, the Father Almighty, and in one Lord Jesus Christ, the Son of God.” (Against Heresies, Book I. Chapter III. 6.)
Due to the clarity of these quotes, comment is largely unnecessary. While Irenaeus is writing against the heretics of old his words still hold a strong rebuke for the modern semi-modalists who have taken up their mantle.
“The Church, though dispersed through our the whole world, even to the ends of the earth, has received from the apostles and their disciples this faith: [She believes] in one God, the Father Almighty, Maker of heaven, and earth, and the sea, and all things that are in them; and in one Christ Jesus, the Son of God, who became incarnate for our salvation; and in the Holy Spirit, who proclaimed through the prophets the dispensations of God” (Against Heresies, Book I. Chapter X. 1.)
“These have all declared to us that there is one God, Creator of heaven and earth, announced by the law and the prophets; and one Christ the Son of God. If any one do not agree to these truths, he despises the companions of the Lord; nay more, he despises Christ Himself the Lord; yea, he despises the Father also, and stands self-condemned, resisting and opposing his own salvation, as is the case with all heretics.” (Against Heresies, Book III. Chapter I. 2.)
“Since, therefore, this is sure and stedfast, that no other God or Lord was announced by the Spirit, except Him who, as God, rules over all, together with His Word, and those who receive the Spirit of adoption,  that is, those who believe in the one and true God, and in Jesus Christ the Son of God; and likewise that the apostles did of themselves term no one else as God, or name [no other] as Lord; and, what is much more important, [since it is true] that our Lord [acted likewise], who did also command us to confess no one as Father, except Him who is in the heavens, who is the one God and the one Father;–those things are clearly shown to be false which these deceivers and most perverse sophists advance” (Against Heresies, Book IV, Chapter I. 1.)
“And therefore it is right first of all to believe that there is One God, the Father, who made and fashioned all things, and made what was not that it should be, and who, containing all things, alone is uncontained.” (The Demonstration of the Apostolic Preaching)
“This then is the order of the rule of our faith, and the foundation of the building, and the stability of our conversation: God, the Father, not made, not material, invisible; one God, the creator of all things: this is the first point of our faith. The second point is: The Word of God, Son of God, Christ Jesus our Lord, who was manifested to the prophets according to the form of their prophesying and according to the method of the dispensation of the Father: through whom all things were made; who also at the end of the times, to complete and gather up all things, was made man among men, visible and tangible, in order to abolish death and show forth life and produce a community of union between God and man. And the third point is: The Holy Spirit, through whom the prophets prophesied, and the fathers learned the things of God, and the righteous were led forth into the way of righteousness; and who in the end of the times was poured out in a new way upon mankind in all the earth, renewing man unto God.” (Demonstration of the Apostolic Preaching)