John 20:28

My Lord. Gr. ho kurios mou. Thomas uses the title with its highest significance (see on ch. 13:13). Kurios (Lord), in the LXX, translates the Heb. YHWH, the divine name sometimes transliterated into English as Jehovah (see Vol. I, pp. 171-173). By his confession Thomas associated the Being before him with the Jehovah of the OT. Such a confession later apparently became a standard formula of faith (see 1 Cor. 12:3).

My God. Gr. ho theos mou. Theos (God), in the LXX, translates the Heb. ’Elohim, the divine title “God.” In the NT Theos is generally used of the Father (Rom. 1:7; 1 Cor. 1:3; etc.), but here, as in John 1:1 (see comment there), the word ascribes deity to Christ. Though there were many things about the relationship of the Persons of the Godhead that Thomas did not yet understand clearly, his confession was more profound and more far-reaching in its implications than those made earlier by others of the disciples (see, for example, Matt. 16:16).