There are a lot of theological words out there. They're all worth knowing but certainly some more than others. One of those words is justification. Justification is that element of salvation wherein God pardons our sins and accepts us as righteous in his sight by virtue of the righteousness of Christ imputed/credited to us. Even there in my definition of it (which leans heavily on WSC 33) you can see a hint of the means of our receiving this justification — faith. The grounds of our justification is outside of ourselves in the life, death, and resurrection of Jesus. The instrument by which we lay hold of Christ and his righteousness is faith. Hence, we say that justification is by faith.
This reality of justification by faith is a thoroughly biblical one and one affirmed perhaps most strongly in the reformed tradition. My aim here, though, is not to do a general biblical and theological defense of this doctrine. Rather, I aim to investigate one aspect of it. That is its actual necessity by virtue of the doctrine of God. The doctrine of God demands that justification be by grace through faith.
The Goal of God
Good luck trying to investigate the reality of God in two paragraphs! It is the most glorious subject of theology as it ponders the one in whom all reality (physical, spiritual, and conceptual) owes its existence. Still there is one thing about God that particularly comes to the fore when we consider what about him might necessitate justification by faith — his love for his glory.
God does all that he does for his own glory. As J. I. Packer wrote, "What is God’s ultimate end in his dealings with his children? Is it simply their happiness, or is it something more? The Bible indicates that it is the glory of God himself." Of course we would say those two pursuits are harmonious as per WSC 1 but that's not the subject here. God does all that he does for his own glory. God creates for his own glory and God saves for his own glory.
Justification and the Glory of God
With that foundation of God's seeking after his own glory established, we are prepared to consider how justification fits inside all of this. If it is the case that God does all that he does for his own glory then surely he justifies sinners for his own glory. The question then is: what method of justification brings God the most glory? The answer: justification on the grounds of the life, death, and resurrection of Jesus and through the means of faith. In other words, justification by faith brings God the most glory.
Paul himself acknowledges this. He writes in Eph 1: 11-12, "In him we have obtained an inheritance, having been predestined according to the purpose of him who works all things according to the counsel of his will, so that we who were the first to hope in Christ might be to the praise of his glory." The believer hopes in Christ. That is faith. They hope not in themselves and their own efforts but in Christ and his finished work. Paul says that this hoping in Christ is "to the praise of his glory". Because we are justified through hoping in Christ, we bring Christ glory.
Suppose you were playing baseball and your team. It'd been a pretty boring game as far as runs are concerned and it's the ninth inning with the score being 0-0. You're the home team which means you now have the opportunity to score a run in the bottom of the ninth and win the game, avoiding extra innings. Up to the plate at two outs comes the star center-fielder. All your hope for winning the game is laid on the man who then blasts a game-winning home run. Your team wins and it was because of his stellar hitting prowess. But more than that, your hoping in him magnified him. Because you could do nothing but hope in him for a home run, you gave him glory.
That it like what happens when a sinner runs to Jesus Christ for justification. He can do nothing of himself. He is dead in his sin and merits no blessing or kindness from God. He stood cursed under the wrath of God. Yet Jesus Christ, God in the flesh, became a curse for him by dying on the cross in his place. That sinner can do nothing but hope in Jesus. He can do nothing but cling to him and say "Lord, you are my only hope. Save me!" He magnifies the Lord by hoping in him.
Imagine justification was not by faith. Perhaps works alone or faith+works. What then? God is not glorified because the sinner can point to something inside himself to glorify for his salvation. Yet, in justification by faith, God gets all the glory. As Paul writes that salvation is "not a result of works, so that no one may boast" (Eph 2:9) or elsewhere that he declares "far be it from me to boast except in the cross of our Lord Jesus Christ, by which the world has been crucified to me, and I to the world."
Sinners are saved by grace through faith because it glorifies God. And because God does all things for his own glory, he simply could not save sinners any other way than by unmerited favor on the grounds of Christ's finished work. Let us then kill all self-boasting and boast in the cross on which Jesus Christ, our hope of glory, was crushed for our iniquities. Let us then hope all the more in this Jesus whose righteousness has been bestowed on us as blessed robe. Let us glorify God as sinners justified by faith.