This is an interesting phrase because it indicates that Jesus is the source not only of the light of spiritual truth but of the light of all truth.
Clearly, this text tells us that all people receive some light and yet not all people receive and rest in Christ for salvation. Thus, the reference must be to Christ’s granting truth beyond the truth of salvation.
This is how the earliest Christian thinkers understood this text, as pointing to Jesus as the ultimate source of all truth and knowledge. John Calvin echoes this in his comments on today’s passage:
“Men have this peculiar excellence which raises them above other animals, that they are endued with reason and intelligence, and that they carry the distinction between right and wrong engraven on their conscience. There is no man, therefore, whom some perception of the eternal light does not reach.”
Dr. R.C. Sproul writes in his commentary John: ‘All light finds its origin in Jesus Christ, who is the fountain of all truth.’ Whenever people use reason and come to the knowledge of the truth, they are benefiting from Christ’s enlightening work even if they do not realize it. Because Christ is the ultimate source of the light of reason, we can reason with others about our faith, knowing that Jesus may grant them knowledge of saving truth.
Tabletalk, 6 March 2018