Longer Catechism (Orthodox) 3.93

Chapter 3 — Part I — Article I (Of God the Father)

Section 93. Q. How is one God in three Persons? A. We can not comprehend this inner mystery of the Godhead; but we believe it on the infallible testimony of the Word of God. The things of God knoweth no man, but the Spirit of God. 1 Cor. ii. 11.