Paul tells us about the importance of living according to the spirit in 1 Thessalonians 5:5: "You are children of the light and children of the day". The light is considered a gift from God because it is a perfection, being children of light means being children of God, or worshipers and followers of wisdom. God is light, the light as the good is expansive: "This is the message we have heard from Him and announce to you, that God is light, and in Him there is no darkness at all." 1 John 1:5
"Children of the light" also means reborn form the spirit: "No one can see the kingdom of God unless they are born again" John 3:3.
The opposite of light is darkness, which is typical of those who live according to the flesh: "We are not of the night, nor of darkness." 1 Thessalonians 5:5.
In the bible comparisons are made between spiritual realities and everyday things in life, the salt, the bread, the light, the yeast. These comparisons facilitate teaching, especially when we try to make understandable very abstract concepts, for example the progressivity of the gifts, or the opposition between the spirit and the flesh (opposition between mentalities). Jesus makes use of this resource by using parables as a teaching method.
Light and darkness symbolize our actions, when we do good we act in full view, the good is beautiful; when we do evil we try to hide it because it is something negative, ugly. Jesus expresses this in Matthew 5:14-16: "You are the light of the world. A town built on a hill cannot be hidden. Neither do people light a lamp and put it under a bowl. Instead they put it on its stand, and it gives light to everyone in the house. In the same way, let your light shine before others, that they may see your good deeds and glorify your Father in heaven."
Paul in other words, when talking about the opposition between light and darkness, is expressing that we have to choose between good and evil, between the spirit and the flesh.
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