Some Jewish people object to Jesus because they think that his followers believe in three gods. Jewish people, of course, believe that God is one and affirm that belief in the synagogue prayer, the Shema, taken from the Book of Deuteronomy.
Hear, O Israel: The Lord our God, the Lord is one. (Deuteronomy 6:4, NIV)
It’s important to know that the New Testament reaffirms this cardinal Jewish belief:
One of the teachers of the law came and heard them debating. Noticing that Jesus had given them a good answer, he asked him, “Of all the commandments, which is the most important?”
“The most important one,” answered Jesus, “is this: ‘Hear, O Israel, the Lord our God, the Lord is one. Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’ The second is this: ‘Love your neighbor as yourself.’ There is no commandment greater than these.”
“Well said, teacher,” the man replied. “You are right in saying that God is one and there is no other but him. (Mark 12:28–32, NIV)