St. Clement of Alexandria (3rd century):
This is the one grace of illumination, that our characters are not the same as before our washing.
...
Being baptized, we are illuminated; illuminated, we become sons; being made sons, we are made perfect; being made perfect, we are made immortal. “I,” says He, “have said that you are gods, and all sons of the Highest.” This work is variously called grace, and illumination, and perfection, and washing: washing, by which we cleanse away our sins; grace, by which the penalties accruing to transgressions are remitted; and illumination, by which that holy light of salvation is beheld, that is, by which we see God clearly.
How can it get more clear?
Here he explicitly ties "illumination" to the change of "character". He speaks of multiple effects of Baptism, a washing and a change of character.
Then he goes on to define "illumination" as that "by which ... we see God clearly."
Illumination, the
lumen gloriae, i.e. the Beatific Vision by which we "see God clearly" is tied directly to the change of character.
St Clement actually adds a third component to Baptism, distinguishing between the "washing," defined as a cleansing away of our sins, and then a separate thing he calls "grace," which he defines as remitting the temporal punishment due to sin as well ("penalties accruing to transgressions are remitted").