Presbyterian theology unreservedly affirms the doctrine of the Trinity as expressed in the Nicene Creed and the Westminster Confession of Faith. The Westminster Confession (Chapter II) teaches that there is but one living and true God, and that in the unity of the Godhead there are three persons — Father, Son, and Holy Ghost — of one substance, power, and eternity. The Westminster Larger Catechism (Q&A 9–11) develops the doctrine in detail, explaining the eternal generation of the Son and the eternal procession of the Spirit.

The Westminster Standards treat Trinitarian orthodoxy as the essential framework for the whole of Christian doctrine. Covenant theology — the organizing principle of Westminster theology — is Trinitarian in structure: the eternal covenant of redemption (pactum salutis) is an agreement within the Trinity, in which the Father appoints the Son as mediator and the Spirit applies redemption. This intra-Trinitarian grounding for salvation is distinctive to Westminster Calvinist theology.