For the uninitiated, the impression could be that Christianity believes in three Gods—the Father, the Son, and the Holy Spirit. But that’s not the case. Christians believe that there is only one God. The doctrine that explains the concept of how the Father, Son, and Holy Spirit are one God is called the doctrine of the Trinity.
This doctrine explains that God has always existed as three persons in one being, each distinct from the other, the Father, the Son, and the Holy Spirit—and yet one being. Each of the persons is fully God, having all of the attributes and the complete essence of God.
The concept of three persons in one God was not something that was explicitly expressed in the Old Testament, though there are Old Testament verses which infer that there is more than one person in God. (See Genesis 1:26; Isaiah 6:8.) The understanding of the three persons in one God became clearer in the New Testament because of the life, death, and resurrection of Jesus, and the outpouring of the Holy Spirit on the believers. The followers of Jesus came to understand that Jesus is God, but He is distinct from the Father, and that the Holy Spirit is also God, but is distinct from the Father and the Son.
In 2 Corinthians, Paul lists the three persons of the Trinity in a manner that shows they are distinct from one another: “The grace of the Lord Jesus Christ and the love of God and the fellowship of the Holy Spirit be with you all” (2 Corinthians 13:14).
Jesus, just before ascending into heaven, commands the disciples to baptize in the name of each person of the Trinity, thus showing He saw them all as equal, all as God. “Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit” (Matthew 28:19).
In truth, the concept of Father, Son, and Holy Spirit being one God is impossible for us as humans to completely understand. That might be disconcerting, but it’s also consistent with our belief that an all-powerful, all-knowing creator God exists. As He reveals Himself to us, it stands to reason that understanding some aspects of Him might be beyond our human experience and understanding. So if you feel you can’t fully understand this, don’t worry about it. The important things are to know that there is one God, that there are three Persons in God, that God loves you and Jesus died for your salvation, and that the Holy Spirit is with you as a helper and counselor.