John Calvin on John 1:8

8. He was not that light. So far was John from needing commendation, that the Evangelist gives this warning, lest his excessive brightness might obscure the glory of Christ. For there were some who gazed so eagerly upon him that they neglected Christ; just as if a person, enraptured with beholding the dawning of the day, would not deign to turn his eyes towards the sun. In what sense the Evangelist employs the word light we shall immediately see. All the godly, indeed, are light in the Lord, (Ephesians 5:8,) because, in consequence of their being enlightened by his Spirit, they not only see for themselves, but likewise direct others by their example to the way of salvation. The apostles likewise are peculiarly called light, (Matthew 5:14,) because they go before, holding out the torch of the Gospel, to dispel the darkness of the world. But here the Evangelist speaks of him who is the only and eternal source of illumination, as he immediately shows more clearly.