Justification at its most basic is straightforward. A man recognizes that he is a sinner. He judges himself to be guilty for his sin before God. He looks for a source whence he can obtain the righteousness he does not possess. He finds it offered to him in the gospel of Jesus Christ. He trusts in Christ, who was raised for his justification, and his faith is reckoned to him for righteousness. He confesses his faith and is saved. It is actually that simple. It is what Paul teaches in Romans 10.5-11 (ESV):
5 For Moses writes about the righteousness that is based on the law, that the person who does the commandments shall live by them. 6 But the righteousness based on faith says, “Do not say in your heart, ‘Who will ascend into heaven?’” (that is, to bring Christ down) 7 “or ‘Who will descend into the abyss?’” (that is, to bring Christ up from the dead). 8 But what does it say? “The word is near you, in your mouth and in your heart” (that is, the word of faith that we proclaim); 9 because, if you confess with your mouth that Jesus is Lord and believe in your heart that God raised him from the dead, you will be saved. 10 For with the heart one believes and is justified, and with the mouth one confesses and is saved. 11 For the Scripture says, “Everyone who believes in him will not be put to shame.”
Matthew Henry brings all this out with great brevity in his comments on that passage, where he also notes the twofold effect of faith: it not only justifies, it also sanctifies, so that the redeemed serve God with both body and soul. In other words, the only kind of faith that justifies is the kind that sanctifies, though it does not justify because it sanctifies. As the Westminster Confession of Faith puts it, “Faith, thus receiving and resting on Christ and his righteousness, is the alone instrument of justification; yet is it not alone in the person justified, but is ever accompanied with all other saving graces, and is no dead faith, but worketh by love” (11.2).
The self-condemned sinner need not perplex himself how this righteousness [of God as Savior] may be found. When we speak of looking upon Christ, and receiving, and feeding upon him, it is not Christ in heaven, nor Christ in the deep, that we mean; but Christ in the promise, Christ offered in the word. Justification by faith in Christ is a plain doctrine. It is brought before the mind and heart of every one, thus leaving him without excuse for unbelief. If a man confessed faith in Jesus, as the Lord and Saviour of lost sinners, and really believed in his heart that God had raised him from the dead, thus showing that he had accepted the atonement, he should be saved by the righteousness of Christ, imputed to him through faith. But no faith is justifying which is not powerful in sanctifying the heart, and regulating all its affections by the love of Christ. We must devote and give up to God our souls and our bodies: our souls in believing with the heart, and our bodies in confessing with the mouth. The believer shall never have cause to repent his confident trust in the Lord Jesus. Of such faith no sinner shall be ashamed before God; and he ought to glory in it before men.