Explaining the Holy Trinity of God the Father, God the Son (Jesus), and God the Holy Spirit will be needed at some point when presenting the God of the Bible to children, Muslims, Jehovah's Witnesses, Mormons and other non-Christians. Yet most Christians have difficulty explaining the Trinity clearly and Biblically.
Part of the reason for this difficulty in explaining the Trinity is that the word "Trinity" is not found in the Bible. Explaining the Trinity to Muslims also must deal with their erroneous equation of the Trinity with polytheism. But the bigger, more common issue is that most professing Christians don't read or know the Bible enough to explain the Biblical truth that the word "Trinity" represents.
The diagram below can be used to explain the Trinity's 'One God in Three Persons' nature:
The
Father is God;
The Son (Jesus) is God;
The Holy Spirit is God;
But,
The Father is not the Son;
The Son is not the Holy Spirit;
The Holy Spirit is not the Father.
Explain that the Bible unequivocally declares monotheism - "Hear, O Israel: The Lord our God, the Lord is one!" (Deuteronomy 6:4) - and that this declaration from the Old Testament was affirmed by Jesus Himself in the New Testament: Jesus answered him, "The first of all the commandments is: ‘Hear, O Israel, the Lord our God, the Lord is one.' " (Mark 12:29)
Next, explain that the Bible also unequivocally declares the deity of both Jesus and the Holy Spirit, and their oneness with God the Father:
In the beginning was the Word, and the Word was with God, and the Word was God... And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth. (John 1:1,14)
Then as He entered a certain village, there met Him ten men who were lepers, who stood afar off. And they lifted up their voices and said, "Jesus, Master, have mercy on us!" So when He saw them, He said to them, "Go, show yourselves to the priests." And so it was that as they went, they were cleansed. And one of them, when he saw that he was healed, returned, and with a loud voice glorified God, and fell down on his face at His feet, giving Him thanks. And he was a Samaritan. So Jesus answered and said, "Were there not ten cleansed? But where are the nine? Were there not any found who returned to give glory to God except this foreigner?" (Luke 17:12-18)
But a certain man named Ananias, with Sapphira his wife, sold a possession. And he kept back part of the proceeds, his wife also being aware of it, and brought a certain part and laid it at the apostles’ feet. But Peter said, "Ananias, why has Satan filled your heart to lie to the Holy Spirit and keep back part of the price of the land for yourself? While it remained, was it not your own? And after it was sold, was it not in your own control? Why have you conceived this thing in your heart? You have not lied to men but to God." (Acts 5:1-4)
Jesus answered them, "I told you, and you do not believe. The works that I do in My Father’s name, they bear witness of Me... I and my Father are one." (John 10:25, 30)
If the hearers acknowledge the passages above but protest that they still can't comprehend how there is one God in three persons, acknowledge that the Trinity is hard to grasp but ask if they ultimately should believe what God said or what they can comprehend. The former trusts God as God; the latter reduces God to what fits into the limited human mind.
As an analogy, if a child flying in an airplane asks how such a heavy machine can remain in the air and then fails to comprehend the aeronautics explained by an engineer, should the child accept the reality of aeronautics or reject it because it is beyond his comprehension?
If Muslim hearers protest that their Koran denies there being one God in three persons, advise them that Muhammad, who dictated the Koran, was illiterate and therefore had never read the Bible, and did not even understand who the three Persons are: instead of God the Father, Jesus and the Holy Spirit, Muhammad mistakenly thought that the Trinity refers to Allah, Mary and Jesus:
And [beware the Day] when Allah will say, "O Jesus, Son of Mary, did you say to the people, 'Take me and my mother as deities besides Allah?'"... - Quran 5:116.
Did Jesus ever tell anyone to treat Mary as a deity?
Of course not. To the contrary, Jesus told people that Mary had nothing to do spiritually with Him and God the Father:
While He was still talking to the multitudes, behold, His mother and brothers stood outside, seeking to speak with Him. Then one said to Him, "Look, Your mother and Your brothers are standing outside, seeking to speak with You." But He answered and said to the one who told Him, "Who is My mother and who are My brothers?" And He stretched out His hand toward His disciples and said, "Here are My mother and My brothers! For whoever does the will of My Father in heaven is My brother and sister and mother." (Matthew 12:46-50)