Eidos denotes the visible form, shape, or appearance of something — what can be perceived with the eyes. In the New Testament it appears in the account of the Transfiguration ('the appearance of his face changed' — Luke 9:29), in the descent of the Spirit 'in bodily form (eidos) like a dove' (Luke 3:22), and in Paul's contrast between walking by faith and walking by sight (2 Corinthians 5:7).
The theology of eidos is the theology of visible revelation. At the Transfiguration, the eidos of Jesus' face changed — the divine glory briefly broke through His human form. This foreshadows the resurrection body in which the inner divine reality will fully shine outward (Philippians 3:21). Meanwhile, 2 Corinthians 5:7 ('we live by faith, not by eidos/sight') calls believers to trust the unseen reality — the invisible God and the promised resurrection — rather than limiting trust to what can presently be observed. The visible forms at baptism (dove) and transfiguration are gracious accommodations, not the full picture.