Christ, our righteousness, is the Sun, justification, its light, sanctification, its heat. The Sun is at once the source of both, so that light and heat are inseparable. But only light illumines and only heat warms, not the reverse; both are always present, without the one becoming the other. (3.11.6)I love this, because though it's not up to date with modern physics, it shows so many great biblical truths so clearly.
First, justification is distinct from sanctification. Roman Catholic theology says that being declared right(eous) and accepted before God (our justification) depends on how much we've repented of self-love and turned to love God and others during our lifetime (our sanctification). Against this, evangelical theology says, along with the Bible, that as soon as God says "Let there be light!" and makes the light of Christ shine in us (2 Corinthians 4:6), we are fully and irrevocably reconciled to God. When you're in the sunlight, its brightness is always the same. From our first moments of repentance and faith in Christ, we're fully and unwaveringly right with God and accepted by him.
Second, sanctification is inevitable for the justified. Though the sun's brightness may not vary, the more you're out in its rays, you begin to feel its warmth and heat. You grow warmer yourself because of the sun's continued effects upon you. Therefore the longer you live by faith in Christ, you become more and more aglow with his presence. "And we all, with unveiled face, beholding the glory of the Lord, are being transformed into the same image from one degree of glory to another" (2 Corinthians 3:18).
Third, both aspects of this salvation are encompassed by union with Christ. Both justification (God declaring a sinner innocent and fully in good relational standing before him) and sanctification (God's progressive work of making a sinful person more like him) belong to being in the light. Without the sun, we lack both light and heat; with the sun, we possess both. As Richard Gaffin puts it, "There is no partial union with Christ, no sharing of only some of his benefits. If believers do not have the whole Christ [which they do!], they have no Christ; unless they share in all of his benefits they share in none of them."* Or, in the words of the Belgic Confession (article 22),
For it must necessarily follow
that either all that is required for our salvation
is not in Christ or,
if all is in him,
then he who has Christ by faith
has his salvation entirely.
Finally, you need to be in the light to have this union with Christ. This all assumes we are no longer living in darkness. Both to stand accepted before God and to experience inner renewal, there must come a moment--one great beginning that never ends--when you throw open the door and let in the light of Jesus. And when you do so, you'll realize that it was God himself who "has shone in our hearts to give the light of the knowledge of the glory of God in the face of Jesus Christ" (2 Corinthians 4:4, 6) and has "delivered us from the domain of darkness" (Colossians 1:12-14).
*Richard B. Gaffin, Jr. "Biblical Theology and the Westminster Standards," The Practical Calvinist: An Introduction to the Presbyterian and Reformed Tradition, ed. Peter Lillback (Christian Focus, 2002), p. 439.