That the doctrine of the Holy Trinity is clearly taught in the New Testament is a fact readily admitted by all Christians. The Unitarians, who deny even the New Testament proof for the Trinity, are outside the pale of the Christian Church. The Holy Trinity is revealed: a) in the solemn formula of Baptism given by Christ, Matt. 28, 19, in which the three Persons of the Godhead are represented as equal in authority, dignity, and essence; b) in the wonderful theophany at the baptism of Christ, Matt. 3, 16. 17, where the three Persons of the Godhead were distinctly manifested; c) in the inspired benediction of St. Paul, 2 Cor. 13, 14, where the spiritual blessings of the Three Persons are expressly named. The passage 1 John 5, 7 is too doubtful to be used as a proof for the Holy Trinity. It is said that Cyprian (t 258) quotes it in his work De Unitate Ecclesiae: "Et iterum de Patre et Filio et Spiritu. Sancto scriptum est: 'Et tres unum est.' " Nevertheless it is best to disregard this passage altogether as a proof-text for the doctrine of the Holy Trinity.
However, while all Christians admit the proofs for the Trinity as given in the New Testament, it has been claimed that the Old Testament, while containing indications and traces (indicia et vestigia) of the Holy Trinity, does not exhibit the doctrine so clearly that it could be believed or taught on the basis of the Old Testament passages (Calixtus; modern theologians). To this charge our dogmaticians replied (Gerhard, III, 218): "We do not say that in the Old Testament there is the same clearness and evidence of the testimonies concerning the Trinity as in the New Testament; but we assert that from the Old Testament some testimonies, in exhibiting the doctrine of the Trinity, both can and ought to be cited, since God always from the beginning revealed Himself thus in order that the Church at all times might acknowledge, worship, and praise Him ... as three distinct Persons in one essence." Doctr. Theol., p. 157 As a matter of fact the Old Testament contains not only mere "indications" of the Holy Trinity, but clear passages, in which the doctrine is unmistakably set forth. Such passages are those: a) in which God speaks of Himself in the plural number, Gen. 1, 26; b) in which the Lord speaks of the Lord, Gen. 19, 24; c) in which the Son of God is expressly named, Ps. 2, 7; d) in which three Persons of the Godhead are distinctly enumerated, Gen. 1, 1. 2; 2 Sam. 23, 2; Ps. 33, 6; Is. 42, 1; 48, 16. 17; 61, 1; e) in which the name Jehovah or God is thrice repeated in the same relation, N urn. 6, 24--26; Ps. 42, 1. 2; Is. 33, 22; Jer. 33, 2; Dan. 9, 19; f) from the trisagion of the angels, Is. 6, 3 : g) from the passages in which the Angel of the Lord (יְהוָה מַלְאַךְ) is identified with God, Gen. 48, 15. 16; Ex. 3, 1-7; h) from the reference of Christ to the Old Testament when He proved the true deity and divine personality of the Son of God, Matt. 22, 41--46 compared with Ps. 110, 1. Certainly no one has ever been saved who did not believe in the true God (the Triune God) and the true Savior of the world (the Second Person of the Godhead), since this truth is stated so clearly in Scripture, Acts 4, 12; John 5, 23; 1 John 2, 23. Nor is the plan of salvation which is taught in the New Testament different from that which was taught in the Old Testament, Rom. 3, 21-24; 4, 1-3. We rightly hold therefore that the doctrine of the Holy Trinity is so clearly set forth in the Old Testament that the believers in the Old Testament most assuredly had a true knowledge of God and of the promised Savior, His beloved Son.