Book 1 - Knowledge of God the Creator
Chapter 13 - One essence of God, which contains Three Persons
Section 16-20 - Distinction and unity of the three Persons
Section 18 - Difference of Father, Son and Spirit
We have to be careful using human pictures here.
The Father is the source of activity.
The Son is the wisdom and order of activity.
The Spirit is the power of activity.
The Father begets the Son, eternally.
The Spirit proceeds from both Father and Son.
Romans 8:9, 11 proves the last truth.
Spirit of God and Spirit of Christ are the same.
Section 19 - Relationship of Father, Son and Spirit
The Father is not the Son.
The Father is in the Son; the Son is in the Father (John 14:10).
Each Person is God with respect to Himself, and Father, Son, or Spirit with respect to the other Persons.
Section 20 - The Triune God
When we say "God" we refer to all three Persons, not to any one in relation to the other Persons.
Jesus is not another God than the Father.
Yahweh corresponds to Christ, since the apostles substitute Lord for Yahweh (2 Cor 12:8-9; Joel 2:32=Acts 2:21).
God's essence is spirit (John 4:24).