In the second edition of his Systematic Theology, the Evangelical theologian Wayne Grudem writes:
Wayne Grudem - Systematic Theology, Chapter 14
John 20:28 in its context is also a strong proof for the deity of Christ. Thomas had doubted the reports of the other disciples that they had seen Jesus raised from the dead, and he said he would not believe unless he could see the nail prints in Jesus' hands and place his hand in his wounded side (John 20:25). Then Jesus appeared to the disciples when Thomas was with them. He said to Thomas, "Put your finger here, and see my hands; and put out your hand, and place it in my side. Do not disbelieve, but believe" (John 20:27). In response to this, we read, "Thomas answered him, 'My Lord and my God!'" (John 20:28). Here Thomas calls Jesus "my God." The narrative shows that both John in writing his gospel and Jesus himself approve of what Thomas has said and encourage everyone who hears about Thomas to believe the same things that Thomas did. Jesus immediately responds to Thomas, "Have you believed because you have seen me? Blessed are those who have not seen and yet have believed" (John 20:29). As far as John is concerned, this is the dramatic high point of the gospel, for he immediately tells the reader - in the very next verse - that this was the reason he wrote it:
Now Jesus did many other signs in the presence of the disciples, which are not written in this book; but these are written so that you may believe that Jesus is the Christ, the Son of God, and that by believing you may have life in his name. (John 20:30-31)
Jesus speaks of those who will not see him and will yet believe, and John immediately tells the reader that he recorded the events written in his gospel in order that they may believe in just this way, imitating Thomas in his confession of faith. In other words, the entire gospel is written to persuade people to imitate Thomas, who sincerely called Jesus "My Lord and my God." Because this is set out by John as the purpose of his gospel, the sentence takes on added force.
Here, Grudem takes the standard Trinitarian understanding of John 20:28, wherein Thomas is confessing Jesus to be Yahweh. This understanding of John 20:28 is a misreading, and is addressed in this article.
From this claim, Grudem proceeds to argue that teaching that "Jesus is Almighty God" - which is how he understands Thomas's confession - is why John says he wrote his Gospel.
In response, notice that John specifically states the content of the faith that he is writing his Gospel to persuade people of - "that Jesus is the Christ, the Son of God". If one intends, above all things, to persuade people that Jesus is God, then they would mention such a thing here, rather than the lesser positions of "the anointed of God" (Christ) and "Son of God". The fact that John makes this content specifically his main thesis demonstrates that he did not understand Thomas's confession to be that Jesus is Almighty God, nor did he understand anything else in his Gospel to be teaching that.
Grudem's statement that "the entire gospel is written to persuade people to imitate Thomas, who sincerely called Jesus 'My Lord and my God'", is true, when properly understanding that Thomas's confession is equivalent to recognizing Jesus as "the Christ, the Son of God". However, it is not true in Grudem's Trinitarian understanding of John 20:28. John had dozens of pages to teach the doctrine that Jesus is God, and he never does so. Instead, he taught his aforementioned thesis - that Jesus is the Christ (John 1:17, 1:41, 3:28, 4:25-26, 6:69, 7:31, 11:27, 17:3). Consider these verses, and consider how clearly and unambiguously John communicates that Jesus is the Christ. Then, contrast that with the verses brought forward to allegedly show that Jesus is Almighty God, and notice that they are always contested, or unclear, or how their alleged messages are deduced, rather than actually stated.
In summary, Grudem magnifies his misunderstanding of John 20:28 in his interpretation of John 20:30-31. He not only mistakes what Thomas is confessing, but then asserts that his misunderstanding is the crux of John's Gospel, and the main point that John intended for people to understand, even when John is explicitly saying something completely different in the passage that Grudem is using, which when properly understood, actually corrects Grudem's misunderstanding of Thomas's confession.