As Christ’s disciples we are called to “love one another as He has loved us,” and it is by this that all men will know that we are His disciples (John 13:34-35). What does His love for us look like, and how are we to imitate it? It looks like the father who saw his son while he was still far off, “was filled with compassion, and ran and put his arms around him and kissed him” (15:20). But to truly appreciate the significance of the father’s reception of his repentant son, we need to look at what he did next: he got a robe—the best one—and put it on him (15:22). And so the son entered his father’s house adorned in his father’s “best robe.” When he walked into the house, all who saw him saw what he was wearing. His clothing identified him as his father’s son. Likewise, we who have been reconciled to God through the death of His son (Romans 5:10) are now holy and blameless in His sight (Colossians 1:22). We are now “found in Him, not having a righteousness of our own that comes from the law, but one that comes through faith in Christ” (Philippians 3:9). As John Calvin states, “To be justified in God’s sight is to be reckoned as righteous in God’s judgment and to be accepted on account of that righteousness...having been clothed in [Christ’s righteousness, we appear] in the sight of God not as a sinner, but as a righteous person...Our righteousness is not in us, but in Christ. We possess it only because we participate in Christ; in fact, with him, we possess all his riches.” Like the father in the parable who put his “best robe” on his son, our Father has clothed us with the garments of salvation, and put a “robe of righteousness” on us (Isaiah 61:10). It should not be overlooked that this is the very same passage Jesus read in the temple at Nazareth when He said, “Today this scripture is fulfilled in your hearing” (Luke 4:21). The “robe of righteousness” we wear is the righteousness of Christ. This is why the prophets call Him “the LORD our Righteousness” (Jeremiah 33:16). Therefore, to love one another as He has loved us is to view one another as He views us: as clothed in the righteousness of Christ. When we walk into the house, every other member of the house should see what we are wearing. And when we begin to see one another as clothed in Christ’s righteousness, it will change the way we treat one another, even as it changes the way we see ourselves, because we are looking at one who appears before God exactly as we do—“holy and blameless before Him in love” (Ephesians 1:4).
Those in the church who view others not according to Christ’s righteousness, but according to comparisons of outward performance that they set up (2 Corinthians 10:12), and judge them, do so because they have not yet learned to view themselves according to God’s righteous judgment (Romans 2:5). They may, like the older son in the story, believe themselves to be deserving of God’s favor based on their own merit. They may view themselves as “rich, and in need of nothing,” but in reality apart from Christ they are “wretched, pitiable, poor, blind and naked” (Revelation 3:17). They are, in fact, what the younger son knew himself to be. They may have attempted to “cover themselves with their works,” but all their “righteousnesses” appear as “filthy rags” (Isaiah 59:6; 64:6). God invites them to “buy” (“without money and without price,” cf. Isaiah 55:1-3) a “white garment” so that the “shame of their nakedness” no longer appears (Revelation 3:18; cf. Genesis 3:10); but before they can “buy” it, they have to come to the place of knowing they have nothing of their own with which to pay for it. It is “not by works of righteousness that we have done, but by His mercy that He has saved us” (Titus 3:5). The Pharisees in the first century, like the older son in the story, could not accept this. To the degree that those in the twenty-first century church cannot accept it, and rely in any part on their own efforts or merits to secure their place at the table in God’s kingdom, they too will be left outside. Their attempts to “enter the kingdom by force” (Luke 16:16) will result in judgment of others, in order to make themselves appear more righteous by comparison. But they will not appear before God clothed in Christ’s righteousness, until they come to understand that their need for His mercy is as great as that of any other sinner whom they have judged.
I am reminded of Jesus’ words to Simon the Pharisee, regarding the woman who wept at His feet: “I tell you, her sins, which were many, have been forgiven; hence she has shown great love. But the one to whom little is forgiven, loves little" (Luke 7:47). By this Jesus was not saying that this woman needed more forgiveness than others. He was saying that only when we are aware of the extent of our need for forgiveness are we able to receive it, and display it to others in gratitude for the mercy we have been shown. The older son in the parable was focused on what he deserved in comparison to what his younger brother deserved, because of the actions they had each performed. But what significance do any of these acts of service we perform have apart from gratitude to God for His mercy? It is the love of Christ that compels us (2 Corinthians 5:9). We prove that He has loved us by displaying who we are in Him to one other, by loving one another as He has loved us. We reciprocate God’s love for us, and display the mercy He has shown us, by loving and forgiving and being merciful toward one another. This is how “all men will know that we are His disciples” (John 13:34). But we cannot show mercy, until we have known mercy. In other words, we cannot love much, until we have been forgiven much. If we only think we needed to be forgiven little, we will only love little, and never according to the exceeding riches of God’s grace and mercy, by which He has greatly loved us in Christ Jesus (Ephesians 2:4-7).
The Bible tells us that in Christ we have been reconciled to God and to one another: “Remember that you were at that time without Christ, being aliens from the commonwealth of Israel, and strangers to the covenants of promise, having no hope and without God in the world. But now in Christ Jesus you who were once far off have been brought near by the blood of Christ... So then you are no longer strangers and aliens, but you are citizens with the saints and also members of the household of God” (Ephesians 2:12-13,19). We who were once far off have been brought near, and are now members of God’s household. Like the father in the story who ran to his wayward son, our Father has run to us, and bestowed upon us all the wealth of His house. How then should we treat one of our brothers and sisters, even when they fall? Since God has reconciled us to Himself through Christ, He has given us the ministry of reconciliation toward one another (2 Corinthians 5:18). We are called to restore one who is over-taken in any fault, in a spirit of gentleness (Galatians 6:1). But just as the older son’s self-righteousness prevented him from welcoming his brother home and joining the family celebration, and just as the Pharisees’ self-righteousness prevented them from eating at the same table with “sinners” and receiving a place at that table themselves; our self-righteousness and judgment will prevent us from ministering healing and restoration to our brothers and sisters when they fall, and also from experiencing the unity of the faith and the fullness of Christ (Ephesians 4:13). But when we restore those who fall by reminding them of the righteousness of Christ that clothes them, and identifies them as children of God, we proclaim and celebrate the good news of the kingdom of God: Once we were far off, but now we have been brought near. “Once we were not a people, but now we are God's people; once we had not received mercy, but now we have received mercy” (1 Peter 2:10). Indeed, God’s promised mercy to His people has been fulfilled in us, and we prove that every time we show it to one another.
 Alister E. McGrath, ed., The Christian Theology Reader, 3rd ed. (Wiley-Blackwell, 2011), 449.