“How can God and Christ and the Holy Spirit be one person?”
I don’t believe that the Father, Son and Holy Spirit are one person. I believe They are “one God”. There is a difference. “Hear, O Israel! The LORD is our God, the LORD is one! You shall love the LORD your God with all your heart and with all your soul and with all your might” (Deuteronomy 6:4, 5 – New American Standard Bible). The Bible speaks of the “Deity” (Colossians 2:9), which Jesus Christ represented in bodily form on earth (Hebrews 2:12 – 15). The word for the “name” of God is “the LORD” – and is so translated in the New American Standard Bible (see above). However, the word for “God” is different.
The Hebrew word for “God”, which is used extensively in the Old Testament, is the word “ELOHIM.” This is a word similar to “group” or “team” – a word that indicates a unity of more than one person. It is used in the creation account of Genesis 1. “In the beginning God created the heavens and the earth…Then God said, “Let Us make man in Our image” (v 1, 26). Notice the plural pronouns, “us” and “our” in Genesis 1:26. There is “one God,” who uses “us” and “our” with reference to Himself!
I don’t know that we will ever (on this side of eternity) understand the concept of the trinity. But that is exactly what Jesus describes in Matthew 28:19, “Go therefore and make disciples of all the nations, baptizing them in the name of the Father and the Son and the Holy Spirit”. The apostle Paul wrote to the church at Corinth, “May 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).
The “Deity” of the Bible is composed of the Father, the Son and the Holy Spirit. While Jesus was on earth, He often prayed to the Father in heaven (Matthew 6:9; John 17:1; etc.). Notice His promise to the apostles, “If you love Me, you will keep My commandments. I will ask the Father, and He will give you another Helper, that He may be with you forever; that is the Spirit of truth” (John 14:15 – 17a). There you find all three members of the Deity. Jesus (on earth) would pray to the Father (in heaven) for the Holy Spirit (in heaven) to be sent to earth to guide the apostles. So, while there are three “Persons” in the Deity, they are still only “one God.“