Modalism is Heresy
Modalism is quite possibly the most common heresy/error in the Church concerning the nature of God. It, in its most basic form, is a denial of the Trinity, which states that there is One God Eternally existing as three persons. In the OT God was manifest as the Father, in the NT/Gospels as the Son, and after the ascension as the Spirit. Modalism denies the distinctiveness of the persons of the Trinity/Godhead while maintaining the Divinity of Christ.