"...God is light, and in him is no darkness at all." (1 John 1:5, NASB)
The light always vanquishes the darkness. If you bring a light into a dark room, it is not the light that disappears, but the darkness. Even a dim flashlight overcomes the darkness that its light comes in contact with. In every case, the darkness diminishes and the light becomes brighter.
This makes the spiritual truth of this verse so very urgent. Imagine what will happen to those living in darkness at the judgment, when they face the source of all light, God Himself. This is why the Christian life is a life of sanctification and a call to holiness.
Jesus is our redeemer and perfector. If for you He is not the latter, then for you He was never the former. We must have Jesus not only as a free ticket into heaven, but also as our surgeon to systematically cut out the darkness within us and replace it with His light. Because, at the end of the day, we will either have Him vanquishing our darkness from within and transferring us into greater degrees of light (or glory, see 2 Cor. 3:18), or we ourselves will be vanquished along with all darkness at the great day of judgment. The Light ALWAYS vanquishes the darkness.
We are not saved because we are sanctified. We are being sanctified because we are saved. Christ always finishes what He starts. The Light lives within every believer and is active to root out and eliminate our sin so we may be presented holy and blameless before God.
If the glory of God is to be our light in eternity (Rev. 21:22), then we must be, and we will be, perfected in holiness. If light always vanquishes darkness, how can we hope to stand if any darkness remains in us?
Are you in Christ? Are you submitting yourself to His perfecting work?
Two Side Notes
1. The fact that the brightness of the light is made more evident by the diminishing of the darkness backs up a significant theological truth. Not only is God glorified by the redemption, sanctification, and glorification of His people. He is also glorified by the judgment and condemnation of His enemies. Truly in everything God is glorified!
2. If, in the midst of Christ perfecting us in holiness, we are tempted to think too highly of ourselves, we must remember this. The light we possess is given to us graciously by God. In comparison to Him, it shines from us as a single lit candle in the middle of the noonday sun.