How do you know what people thought was light 2000 ago? The early Christian scriptures not only showed their views on what "light" was in reference to spirit, they had a much more knowledgeable view of it.
They explain the difference in darkness and light. Just because the orthodoxy the catholic ideology supports (Bible) doesn't mean the knowledge isn't there.
The "light" is the Chrism.
The chrism is superior to baptism, for it is from the word "Chrism" that we have been called "Christians," certainly not because of the word "baptism". And it is because of the chrism that "the Christ" has his name. For the Father anointed the Son, and the Son anointed the apostles, and the apostles anointed us. He who has been anointed possesses everything. He possesses the resurrection, the light, the cross, the Holy Spirit. The Father gave him this in the bridal chamber; he merely accepted (the gift). The Father was in the Son and the Son in the Father. This is the Kingdom of Heaven.- Gospel of Philip
The spirit within is the light. It is mind. Spiritual mind. It see's what those in darkness cannot. It has nothing to do with physical lumins.
Whether the view is more knowledgeable or not is a matter to be discussed. But they certainly viewed light as a spiritual thing. Now, the light of everyday existence was seen as 'contaminated' by the material world, but the light itself was seen as spiritual in essence.
As to how I know how people thought about light at that time period: I have read the philosophical discussions we have from that time and before. Light was not always understood as coming into the eyes from outside, for example. it was seen as non-material and thereby spiritual. It was even seen as divine.