"Modalism is probably the most common theological error concerning the nature of God. It is a denial of the Trinity. Modalism 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; and after Jesus' ascension, the mode is the Holy Spirit.
These modes are consecutive and never simultaneous. In other words, this view states that 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."
https://carm.org/modalism