Despite the standard secular mantra that all religions are the same and teach the same thing, nothing could be further from the truth. All religions do have things in common, like a belief in God, praying to God, holy writings that tell the followers about God, and practices of worship.
But every religion has truly substantial differences in how they answer the questions that matter most: “Who is God?” and “What is God like?” There, the answers are radically different. And the God (or god) described in a religion is what then defines everything else about that religion.
A religion is truly valued and honored when its teachings are viewed as unique and worthy of consideration in their own right.
Christianity is a MONOtheistic religion, which believes in ONE personal God. The three great religions of the world that are monotheistic are Judaism, Islam, and Christianity.
What differentiates Christianity from the others is its belief in the Trinity. The Father is God. The Son Jesus Christ is God. The Holy Spirit is God. But they are not three gods, they are ONE God in THREE persons.
Yes, it is complicated. Here it is unpacked with more detail.
Not THREE (or more) Gods.
That is tritheism or polytheism.
That is Hinduism and the ancient pagan pantheons, like the Roman gods, Norse gods, or Egyptian gods.
Christianity is not three Gods named Father, Son, and Holy Spirit.
Not a God that is an impersonal force, power, or energy.
That is Buddhism and Star Wars (the Force).
Not ONE God that is ONE Person.
That is Judaism – Yahweh or Jehovah.
That is Islam – Allah.
Not ONE God with THREE extensions.
Like the three leaves of a clover.
Not ONE God in THREE roles.
Like a man who is a son, husband, and father.
Or a woman who is a daughter, wife, and mother.
The Trinity is ONE God in THREE persons.
The Father, Son, and Spirit are all God.
The Father, Son, and Spirit are all eternal, separate, and individual persons.
This is the Trinity taught, but not fully explained. That is what the Bible does. The Bible teaches the Trinity, but it does not theologically explain the Trinity. The Bible references the Trinity (like in John 1:1-5, 14; Matthew 28:18-20; and 2 Corinthians 13:14). Jesus aroused major opposition from the Jews who believed he was committing blasphemy when he made statements that he was equal with God (see them throughout the Gospels).
And the Apostle Paul had to clearly teach that Jesus was God as he did his missional work among both Jews and Gentiles (Colossians 1:15-20; 2:9).
We believe in God the Father Almighty, Maker of heaven and earth, and in Jesus Christ, His only Son, our Lord … and in the Holy Spirit.
For more theological substance, you can read this article. It is a very good article that goes sufficiently into this vital Christian belief, but is still pretty accessible and understandable.