The 9th Question of the Baptist Catechism involves the trinity, that is, the triunity of God. Let us look, first, at the question and answer, and then, second, at the implications of a trinitarian view of God.
Q. 9: How many persons are there in the Godhead?
A. 9: There are three persons in the Godhead, the Father, the Son, and the Holy Spirit; and these three are one God, the same in essence, equal in power and glory.
(1 John 5:7; Matthew 28:19)
As we just learned through questions 1, 7, and 8, God factors into the Catechism's teaching, and our understanding, at the foundational level. There is one God, only God, as the Scriptures teach. "Hear, O Israel: The LORD our God, the LORD is one" (Deut. 6:4, ESV).
Question 9, however, asks about persons. The 2LCF helps clear up our possible confusion (which, if we are honest, will be much, for we are finite creatures and He is infinite). Chapter 2 is "Of God and of the Holy Trinity," and we read these words in paragraph 3, "In this divine and infinite Being there are three subsistences, the Father, the Word (or Son), and Holy Spirit..." Subsistence is an uncommon word, but when the Catechism was authored the word was used to denote the different persons in the Godhead. This is what we refer to as the Trinity.
There is God the Father, God the Son, and God the Holy Spirit. These three persons, or subsistences, are God, and only one God. As with most things in church history, the Trinity has received much debate. Blood has been shed, councils held, and books upon books have been written upon this subject. Our purposes are not to delve into this debate. Rather, we seek to understand the collective teachings of the Catechism.
The unity (oneness) and trinity (three-in-one) are taught in this answer. But there is additional clarification offered. This information, based upon the teachings of Scripture, helps clear up confusion further and carefully and succinctly dismantle arguments that are heretical.
These three persons are "one God, the same in essence, equal in power and glory." Without getting too philosophical, I find van Mastricht's words helpful in defining essence. He writes, "by the word essence we intend nothing other than what Scripture designates by θεοτης, 'deity' (Col. 2:9)...God exists through essence, not simply a being, but the being par excellence." (Van Mastricht, 2:85) It is, in my own words, His Godness.
What does this matter to everyday lives? Does it change how we live? Does it motivate us to wake up and go to work? Does it affect how we treat our spouses, children, grandchildren, parents, etc.?
If you are feeling particularly nerdy, you can purchase a copy of van Mastricht's work. He offers seven practical benefits of the Trinity.
To answer these questions, I turn to Joel Beeke and Paul Smalley, authors of Reformed Systematic Theology. They write, "The doctrine of the Trinity informs Christian spirituality in a number of ways. It traces a gospel pathway of communion with God via the Mediator: fellowship with the Father through the Son in the Spirit. It opens up the possibility of distinct communion with each person of the Trinity. And it brings an unspeakable fullness of glory that captures the whole person because the Christian's communion is with the one and only God." (Beeke and Smalley, 945)