Berean Research has the answer:
Modalism was declared a heresy by the early church centuries ago, but it keeps popping up like a Whack-A-Mole game. Unfortunately, Modalism is rearing its head once again in our time. It is a growing theological error within evangelicalism that twists the very the nature of God revealed to us through His Word, by denying the Trinity.
Rather than God being three distinct persons, Modalism states that God is a single person who, throughout biblical history, has revealed Himself in three modes or forms. In other words, God in the Old Testament was Father God. Then He became Jesus 2,000 years ago. At Pentecost, God morphed into the Holy Spirit.
you have no problem with denying the doctrine of the Trinity, yourself this: To Whom was Jesus praying in the garden? And what about all those other times Jesus was doing what His Father was doing? (There are hundreds of questions you should struggle with as you read Scripture.)
Do you see the problem?