The Doctrine of the Trinity is also an Article necessary to be believed for Salvation. That is, this doctrine, with respect to the matter, not with respect to all the terms that are able to be applied in expressing it; is so Necessary, that it is not able to be denied without the loss of Salvation, but ought with sufficient explicitness to be believed and known, according to the Athanasian Creed: “Anyone that wishes to be saved, above all it is necessary that he hold the catholic faith; unless one preserve it whole and inviolate, he will doubtlessly perish forever: now, this is the catholic faith, that we worship one God in Trinity, and the Trinity in Unity.” Our AUTHOR proves this:
α. [From Passages, in which the Knowledge of Christ, the Son of God, beside that of the Father, is set forth as absolutely necessary, John 3:18, etc.] Namely, 1. In the belief of whatever Article eternal Life consists, that is necessary for Salvation: but in the belief of the Article of the Trinity eternal Life consists, John 17:3. Neither is Exception to be taken, that mention is here made of Christ, not as God, but as the Mediator sent by God. For, a. Christ was not able to be Mediator, unless He were True God; nor is He distinguished here from God οὐσιωδῶς/essentially, but from God the Father, by whom He was sent. b. Elsewhere Scripture speaks expressly of Him as True God, 1 John 5:20; etc, and asserts the recognition of Jesus as the Son of God as no less necessary, 1 John 4:15. 2. The negation of whatever Article removes God, keeps man from communion with God, and makes him an ἄθεον/atheist in the world, that article is necessary to know and believe. But such is the Article of the Trinity, 1 John 2:23; Ephesians 2:18, 12. 3. The negation of whatever Article implies certain damnation, that is necessary to know and believe: but John 3:18. Now, the necessary recognition of and belief in the Father and the Son, inculcated in these Passages, does not exclude, but rather includes, the Knowledge of the Spirit: for, just as through the Son we come to a Knowledge of the Father, John 1:18, so οὐδεὶς δύναται εἰπεῖν Κύριον Ἰησοῦν, εἰ μὴ ἐν Πνεύματι Ἁγίῳ, no man can say that Jesus is Lord, but by the Holy Ghost, 1 Corinthians 12:3. Hence the Apostle invokes equally τὴν κοινωνίαν τοῦ Ἁγίου Πνεύματος, the communion of the Holy Ghost, as τὴν χάριν τοῦ Κυρίου Ἰησοῦ Χριστοῦ, the grace of the Lord Jesus Christ, and τὴν ἀγάπην τοῦ Θεοῦ, the love of God, for the Corinthians, 2 Corinthians 13:14. As in the passage cited in Ephesians 2:18 three Persons are also distinctly named: only in the Spirit, through Christ, are we said to have access to the Father.
β. From principal Doctrines and Articles of the Faith presupposing the Trinity. 1. For, with the Trinity denied, the hypostatic Union of the human Nature with the Person τοῦ Λόγου, of the Logos/ Word, falls, and with it the belief of the Incarnation; in which doctrine are also set up for us both God the Father sending His Son into the World, Galatians 4:4, and God the Son taking flesh, John 1:14; Philippians 2:7, and the Holy Spirit forming and sanctifying the human nature of Christ in the womb of the Virgin, Luke 1:35. 2. The same obtains in the doctrine of Satisfaction; in which is found God the Father, demanding the punishments for the sins of the elect from the Surety, Romans 3:25, 26; 8:32; the Son, delivering Himself up for the Elect, so that by the λύτρῳ/ransom of His own blood He might redeem them, Ephesians 5:2; Acts 20:28; the Holy Spirit, granted to the Son without measure to fulfill this work, Matthew 3:16, and whose superabounding gifts fell to the Son as a reward for the suffering endured, Psalm 45:7, through which gifts the merits of the Son are applied to the elect, Romans 8:9; 1 Corinthians 2:12. 3. In Justification the merit of the Son is to be considered, as applied to believers through the gracious sentence of the Father as Judge, the insinuation of which sentence in the hearts of believers is applicable to the Holy Spirit, Romans 3:24; 8:33, 34, 16. 4. Also, it is the Son, who merited Sanctification for the Elect, Ephesians 5:25, 26. It is the Father, who on account of the merity of the Son sends forth the Spirit of Sanctification into hearts, Titus 3:4-6. It is the Spiri, through whom the Father and the Son accomplish this work: whence it is in a particular manner called the Sanctification of the Spirit, 2 Thessalonians 2:13; 1 Peter 1:2; etc.
γ. From the Duties of Faith, Worship, and Invocation, prescribed; such that one may not appropriately worship the True God, unless we know the same with respect to Persons, John 5:23, ἵνα πάντες τιμῶσι τὸν υἱόν, καθὼς τιμῶσι τὸν πατέρα, etc., that all men should honour the Son, even as they honour the Father, etc. And, as in general equal Worship is thus commanded to be offered to the Son and to the Father; so He also sets Himself forth as the object of Faith equal to the Father, John 14:1. But no one can believe upon the Father or the Son, but by the Holy Ghost, 1 Corinthians 12:3; 2 Corinthians 4:13. Indeed, that the honor of religious Faith and Worship is due to the Spirit equally with the Father and the Son, our Lord taught, commanding Baptism into the Name of the Triune God, and in this very thing commending very highly to us the belief and necessity of the Article concerning the Trinity. That the Invocation of the Triune God is also incumbent upon us, we saw above, § 21, 26. And we are not even able to invoke the Father, except in the name of the Son, the Mediator, through the Holy Spirit, who isר֤וּחַ חֵן֙ וְתַ֣חֲנוּנִ֔ים, the Spirit of grace and of supplications, Zechariah 12:10.
δ. Not in vain did God so constantly reveal and inculcate this Doctrine under the Old and New Testaments. Neither without cause were the efforts of Satan continually repeated in order to obscure this doctrine: compare WITSIUS’ Exercitation VI in Symbolum, § 8-13; STAPFER’S Theologicæ polemicæ, tome I, chapter III, section XVI, § 1096-1102, 1144, 1145.
 See Ephesians 2:12: “That at that time ye were without Christ, being aliens from the commonwealth of Israel, and strangers from the covenants of promise, having no hope, and without God in the world (καὶ ἄθεοι ἐν τῷ κόσμῳ)…”  See also John 3:34.