Trinity is a word that, like theology, we do not find in the Bible itself. Nevertheless, like theology, it is no less biblical, because the concept that it summarizes is clearly evident in Scripture, from the first page to the last.
Trinity refers to the nature of God’s existence and is a theological description that distinguishes genuine, biblical Christianity from so many cults and frauds. The name itself could be understood as the combination of the words “triple” and “unity,” and that would just about capture the main idea.
In very basic terms, Trinity refers to God’s three-fold being—the fact that he has always been and forever will be one God who consists, simultaneously and distinctly, in three Persons (Father, Son and Spirit), who are each fully God.
Any definition of Trinity warrants additional explanation of all that it does (and doesn’t) mean. For a next step in understanding the doctrine, I recommend checking out the infographic I put together recently. Here is how I defined the term in that graphic:
God eternally exists as three persons, Father, Son and Holy Spirit, and each person is fully God, and there is one God.
If you haven’t ever read a book on the Trinity, you would do well to read one as soon as possible! I recommend James White’s The Forgotten Trinity or Bruce Ware’s Father, Son, and Holy Spirit; both are excellent places to go for an introduction or refresher.