Modalism is probably the most common theological error concerning the nature of God. It is a denial of the Trinity which states that God is a single person who, throughout biblical history, has revealed Himself in three modes, or forms. Thus, God is a single person who first manifested himself in the mode of the Father in Old Testament times. At the incarnation, the mode was the Son. After Jesus' ascension, the mode is the Holy Spirit. These modes are consecutive and never simultaneous. In other words, the Father, the Son, and the Holy Spirit never all exist at the same time, only one after another. Modalism denies the distinctiveness of the three persons in the Trinity even though it retains the divinity of Christ.
Present day groups that hold to forms of this error are the United Pentecostal and United Apostolic Churches. They deny the Trinity, teach that the name of God is Jesus, and require baptism for salvation. These modalist churches often accuse Trinitarians of teaching three gods. This is not what the Trinity is. The correct teaching of the Trinity is one God in three eternal coexistent persons: The Father, the Son, and the Holy Spirit.