There are a lot of problems I see with that doctrine, but I will make no attempt to address them all in one post. Rather, there are two biblical teachings that I think utterly exclude the possibility of the modalist deity.
First are the descriptions of circumstances in which all three Persons are present at the same time. Consider the account of the baptism of Jesus: "Then Jesus came from Galilee to the Jordan to John, to be baptized by him. John would have prevented Him, saying, 'I need to be baptized by You, and do You come to me?' But Jesus answered him, 'Let it be so now, for thus it is fitting for us to fulfill all righteousness.' Then he consented. And when Jesus was baptized, immediately He went up from
|Sabellius, Founder of Modalism|
Second is what we are told about Jesus after His ascension: "Being therefore exalted at the right hand of God, and having received from the Father the promise of the Holy Spirit, He has poured out this that you yourselves are seeing and hearing" (Acts 2:33; see also 4:31 and 7:55, both of which are after the sending of the Spirit at Pentecost). When He ascended, where did He go? He didn't come back here as the Holy Spirit, as the Modalists claim. Rather, it explicitly states that He is at the right hand of the Father (and thus distinct from Him), from where He sent the Spirit (requiring that they also be distinct). Paul told us even more: "Christ Jesus is the one who died—more than that, who was raised—who is at the right hand of God, who indeed is interceding for us" (Romans 8:34). Where did Jesus go? Again, to the right hand of the Father. What is He doing now? Interceding for us. Paul tells us both where Jesus is and what He is doing right now, and it isn't what the modalists claim! And one final verse to clinch it: "After making purification for sins, He sat down at the right hand of the Majesty on high" (Hebrews 1:3). Again, the Scriptures tell us where the ascended Jesus is, and it isn't where the modalists try to put Him.
That old diagram has it right: the Father is God, the Son is God, and the Holy Spirit is God; however, the Father is not the Son or Holy Spirit, the Son is not the Father or the Holy Spirit, and the Holy Spirit is not the Father or the Son. And our salvation depends on it. If there is no heavenly Son to plead for us before His Father, then the believer has no hope of justification.