Our first consideration, when discussing such a weighty topic, is to have the attitude of Exodus 3:5 where Moses is told: "Take your sandals off your feet, for the place where you stand is holy ground.”
The Trinity doctrine recognizes the biblical revelation of three distinct Persons, The Father, Son, and Holy Spirit, Who are called God. Yet God is "One." How to reconcile this concept is our task, and in doing so we recognize the incomprehensible.
First off, we are monotheistic, Monotheism is the belief in one true God. He alone is the Creator, Sustainer, Savior, and Judge of all His creation. Deuteronomy 4:35 "Unto thee it was shewed, that thou mightest know that the Lord he is God; there is none else beside Him."
Seeking to explain that which is mysterious and incomprehensible begins with the recognition that when dealing with the infinite, that the human mind is finite. And there is nothing more infinite that God Himself, Whom we approach with reverence and dignity and yes boldness, in the name of the Lord Jesus Christ, alone. And our definition must begin with the text of scripture, rather than mere human reason.
The Father is God
John 6:27 Do not labor for the food which perishes, but for the food which endures to everlasting life, which the Son of Man will give you, because God the Father has set His seal on Him.” nkjv
Elect according to the foreknowledge of God the Father
The Son is God
Colossians 2:9 ..in Him dwells all the fullness of the Godhead (Deity) bodily. jnd
John 1:1 In the beginning was the Word and the Word was with God, and God was the Word
John 20:28 Thomas saith unto Him, "my Lord and my God" (directly from the Greek, "the Lord of me, and the God of me")
The Spirit is God
Acts 5:3-4 But Peter said, Ananias, why hath Satan filled thine heart to lie to the Holy Ghost....thou hast not lied unto men, but unto God.
God is ONE
Deuteronomy 6:4 Hear, O Israel: The Lord our God is one Lord:
God is One yet plural/triune:
Genesis 1:1,26 In the Beginning God (Elohim-plural) created the heavens and the earth...let us make man in our own image...
The Father is God, the Son is God, the Holy Spirit is God, yet there is but One God, this is scriptural truth. And we must hold to it without wavering.
Some errant and heretical views men have proposed, in explaining this mystery include:
Arianism - denies the Deity of the Son. Teaches that Christ and the Holy Spirit are creations of the Father and not one in nature with Him.
Modalism - Teaches that God is not 3 distinct persons but reveals Himself in 3 different forms.
Partialism - Asserts the Father, Son and Holy Spirit are each 1/3 of God.
Tri-Theism - A belief in 3 Gods,
Monarchianism - God existed "in" Jesus and in the Holy Spirit, but he latter two aren't God
Patripassianism - God the Father "became" the Son and then the Spirit
One-ness Pentecostalism - (akin to Sabellianism) Teaches One Person in the Godhead rather than three distinct Persons.
The Truth is that that the Father is God, the Son is God, and the Holy Spirit is God...but there is only One God. This simple yet unfathomable statement is the orthodox and biblical position.
Men have tried to put a definition in place, here is a suggestion, but remember this is creedal, and we rest on revelation and scripture, not creeds:
The Trinity is a mystery which cannot be comprehended by human reason, but is only understood through faith. We worship one God in Trinity, and Trinity in unity, neither confusing the persons, nor dividing the substance. We are compelled by the Christian truth to confess that each distinct person is God and Lord. And that the Deity of the Father, Son, and Holy Spirit is One. Equal in Glory and equal in majesty.
Misc. Thoughts on the Trinity:
God is one in Being, and three in Person. We must distinguish between a being and a person. "Being" is that which makes you what you are. "Person" makes you who you are. All human beings share the same kind of "being" in that they are human beings. But each person is not the same as other persons. All are the same in the sense of being human beings. But as persons all are different.
God is one Being, yet three/Triune in Persons. Father, Son, and Holy Spirit.
Some object in this way: Jesus said, "the Father is greater than I." This may pose a question, but we must remember the being is the same (both Father and Son are Deity), yet the roles of each may differ in the Godhead. Roles are different in scope. Even in this explanation we fall short because we are doing our best here to describe Deity that is indescribable, except for that which is revealed in scripture.
The actual first mention of the Trinity, is in Genesis 1:1 In the beginning God created the heavens and the earth. The word for "God" here is plural, Elohim (the plural of God). Yet the word "created" treats it as singular. Plurality and singularity are present in the Godhead in the very first bible verse. We see this further illustrated when God refers to Himself as "we" such as "let Us make man in Our image."
Even the oft quoted revelation of monotheism in the Shema is of interest here. It may be recited twice a day by Jews from Deuteronomy 6:4, "Hear oh Israel, the LORD our God the LORD is One". "Echad" is the word for "one". This word is not used for a singularity, but more like a cluster of grapes, in their singularity as a whole. Even here we see hints of the Trinity, the one cluster of grapes is singular, yet plural. What a revelation that was in human history, in the backdrop of polytheism, to hear such words. Yet we must remember with reverence that depth of Gods character is unlike what we see in the created universe. It is the revelation of Jesus Christ that brings this Trinity matter to our attention more clearly, yet the shades of meaning are very present in the Old Testament. And even when we have viewed what we can as finite beings, in the end we must bow to the full counsel of God, and we see that God is Father, Son, and Holy Spirit. One Being, Three Persons. Tis mystery all, but is not unknown.
-this latter section was adapted & tweaked from online apologetic ministry by Nabeel Qureshi