One biblical text that has long fascinated me is John 20:28. Perhaps this is because it was spoken by my namesake, the apostle Thomas.
Over a decade ago, when I maintained a (now defunct) website entitled 'In Defence of Christadelphian Doctrine,' I wrote an article explaining what I thought this verse means. Some time after that, I developed a more elaborate explanation of the verse, which had Thomas acknowledging the truth of Jesus' claim in John 14:9 (a dialogue in which Thomas actively participated - see John 14:5). One can find another Christadelphian explanation of this text on the Wrested Scripture website.
All of these explanations propose that Thomas was not actually addressing Jesus as his God, but rather as a representative of his God; someone who manifests God's attributes but is not himself God.
Upon further reflection, I now find the above explanations completely unsatisfactory. I believe that in John 20:28, Thomas was actually addressing Jesus as his Lord and God. There are three reasons for this:
1) This interpretation follows a straightforward reading of the text; no verbal gymnastics required!
2) Thomas' emphatic confession functions as the climax of the Fourth Gospel, placed as it is just prior to the summary statement in John 20:30-31. It is the crowning moment in the writer's effort to prove the theological statement made in the Prologue (John 1:1-18). The Word made flesh, who was God, was finally received as God by men. To give the statement an elaborate, cryptic interpretation is to rob the Gospel of its climax.
3) It is absolutely unthinkable that a monotheistic Jew such as Thomas was, cautious as they were about misusing divine names, would address a fellow human being as "my God." It is true that there was some flexibility in the use of the word 'god' (theos in Greek; elohim in Hebrew) so that it could be applied to humans in certain rare circumstances (Ex. 7:1; Ps. 82:6 cf. John 10:34). However, we must not gloss over the vast difference between quoting scriptures in which God refers to humans as gods, and a man addressing another man as "My God!" Thomas would by no means have uttered such rash words, which could easily be mistaken for blasphemy, if he only meant that Jesus represented God. Nor would John have written them down without qualification.
Taking Thomas' words at face value, and interpreting them in light of their historical and literary context, we come inevitably to the conclusion that Thomas confessed Jesus to be his Lord and God because that is who he believed him to be.