The call to discipleship is a call to life which begins with death. Much of the modern Christian message has been stripped of all of its weight because of the fact that the Christians don't know what the Gospel is anymore. Just like when people who don't know what the Scripture teaches about God's nature resort to defining Him by whatever it is they want Him to be like, so likewise when people don't know the Gospel, they conform it to their own nature as well. The writer is perhaps most guilty.
According to Mark's account of Jesus' life, He ended his time with them by telling them to go and preach gospel (see Mark 16:15--"Go and proclaim the gospel to the whole creation"). According to Matthew, this conversation went like this: "All authority in heaven and earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the Name of the Father, the Son, and the Holy Spirit, and teaching them to obey all that I've commanded you. And surely I'm with you always, to the end of the age" (Mt 28:18-20). Finally, Luke has Jesus ending his time with them by saying, "When the Holy Spirit comes upon you, you will bear witness about me" (Ac 1:8). Whereas these differing accounts give slight variation, the variation is certainly only slight, and the message is still the same: "Go and proclaim ME." It's as if Jesus is telling them that their job description is to go and just talk about Him. In fact, the Acts account isn't even a command--it's a declaration: "You WILL proclaim me when the Spirit comes." This is reminiscent of His words to the disciples about their impending persecution in Mt 10:19-20: "Do not be anxious how you are to speak...for it is not you who speak, but the Spirit of your Father speaking through you." He is promising that when "the Spirit of truth" (Jn 14:17, 15:26, 16:13), who is also the "Spirit of adoption as sons" (Rom 8:15), comes upon them, He will also become a "Spirit of power" (2 Tm 1:7) who will then become a "Spirit of conviction" to all they speak to (Jn 16:8, 1 Thes 1:5). The Holy Spirit will, for all intents and purposes, do everything in their ministry.
Whereas the Spirit has regenerated them into new life (see Jn 3:5, 6:63; cf Eph 2:4-5), and so now leads them into the truth (Jn 14:26, 16:13), He will then begin empowering them for ministry. The Spirit draws them to Christ, teaches them the truth, convicts them of sin, and sanctifies them...in other words, the Spirit brings them to become Christians and remain Christians. So also the Spirit will bring about and empower obedience to His call. The Christian who is called by Christ to ministry is never an island--he is always with a calling of which Christ already has the results, and he always has the promise that even if he fails in certain parts, he won't fail finally, because the results are Christ's (see Jesus' prayer for Peter in Luke 22:31-34, and Peter's subsequent fall and return in 22:54-62; also read carefully the language of Paul in Ephesians 2:8-10). As the disciples are sent to go and preach the Gospel, they can be sure that the Lord will bring about fruit...which is good because apart from Him, they couldn't bear any (see Jn 15:5).
What was the Gospel, in simple terms? Paul says in Romans 1:16-17 that the gospel is "the power of God for salvation, to everyone who believes", and then he says that in the Gospel, "the righteousness of God is revealed from faith to faith". So God literally saves people through the message of the Gospel, and in saving them, his righteousness is revealed. Still, it appears abstract.
But this is why the Scripture is so glorious. In 1 Corinthians 1, he defines this Gospel for us: "Christ did not send me to the baptize, but to preach the gospel, and not with words of eloquent wisdom, lest the cross of Christ be emptied of its power" (1:17). He continues, "For the word of the cross is folly to those who are perishing, but to us who are being saved it is the power of God" (1:18). So whereas the Gospel is identified a certain way by Jesus in his commissioning of the disciples (obedience to Him, speaking about Him, etc.), Paul identified this further to help clarify (and so fulfill his mission given him by Christ--see Ac 9:15, 26:15-18): the Gospel is the message of the Cross. In IT the righteousness of God is revealed.
But how is this?
First, beginning with Christ's identity: "the Lamb of God who takes away the sins of the world"--Jn 1:29; He who will "save His people from their sins"--Mt 1:21; "the Word of God...who..became flesh and dwelt among us"--Jn 1:1-3,14, etc. If He who is from God and is the eternal sovereign Creator (see Col 1:15-16, Heb 1:2-4) has to go to the cross in order for anyone to be saved, the gap of righteousness/holiness between God and ourselves must be infinitely wide, so much that only God could span it Himself. God is infinitely holy--"holy" means "set apart", but when speaking of God, it doesn't just mean 'set apart' like the smelly kid at recess is 'set apart' from all the other kids because they don't like him--that kind of 'set apart' is different. The 'set apart'ness of God is one characterized by transcendence (Is 55:8-9), independence (Ac 17:25), and unapproachability (1 Tim 6:16). He is so holy that human eyes can't behold Him in all of his glory. Cross-reference Leviticus 11:44 (God saying "be holy for I am holy") with Matt 5:48 (Jesus saying "be perfect as your heavenly Father is perfect") to see that Jesus defines "holiness" as "perfection". One begins to see the need for God to show grace and mercy--He could never be reached or approached by human effort. It's foolishness to trust in one's own goodness (see the Pharisee in Lk 18:9) especially when there IS no one good or righteous! (See Mk 10:18 and Rom 3:10, 8:7.) If Christ has to come and offer Himself on the cross, and this be the ONLY way to be saved (and it most certainly is: read...the Gospels), how righteous must God be, compared to how unrighteous we must be? In this, the righteousness of God is revealed through the cross. The cross might be foolishness to world, but to those who are saved, it's God's power to save them.
Second, when this is understood--that God is infinitely holy, and mankind, though once good but now sinners addicted to sin and incapable of changing self (see Ecclesiastes 7:29, Jerem 13:23, Jn 8:34)--one sees his utter despair and dependence on divine mercy from God. One sees that salvation/justification is something that he can't do enough for, because God is just that much more holy and good than any "holy" or "good" work a person can accomplish. A person, with the tax collector from the Luke 18 passage cited above, knows that he can't justify himself, and can only stand with his face pointed downward, beating his breast and crying out, "God, have mercy on me, a sinner!"
The only proper place of worship is that which has a proper understanding of the identity of the one being worshiped vs the one worshiping. If a woman worships her beauty, it's inappropriate because beauty is only fleeting and can't truly give her what she needs anyways. If a man worships his favorite sports team, it's inappropriate because sports are simply games where the creators of the game created a goal (a football which needs to be brought to an endzone by a team of players), and then a problem which thwarts the accomplishing of this goal (the other team GUARDING said endzone). See? It's a game. Beauty, while beautiful, is fleeting; sports, while fun, are silly games. And most can't see clearly enough to tell the difference between appreciation and worship. Most women who worship their looks don't know they do, and most men who worship their sports don't know they do (though some know, but are just too lazy and stupid to want to deal with it).
And this is why the Cross matters so much. In the cross, God's righteousness is shown for what it is--perfection and holiness. In the cross, our righteousness is shown for what it really is--non-existent. At least, not before a holy God. The sovereign-Creator-turned-crucified-Savior has become a living sermon--none can be saved apart from Christ.
But then the second tier of God's righteousness at the cross is revealed--the righteousness which God gives to those who don't have any, but believe in Jesus.
--First, note that directly after the "trusting in self-righteousness" Tax Collector and Pharisee parable in Lk 18, Jesus rebukes his disciples for turning away people who brought their infants to him, saying, "Whoever does not receive the kingdom of God like a child shall not enter it" (18:17). Jesus had just told a parable rebuking those who "trusted in themselves, that they were righteous", and now He's rebuking the disciples for turning people away from coming to Him. THEN comes the rich young ruler. When he asks about how to receive eternal life, Jesus tells him to sell all he has, "and come follow ME" (18:22). This story is not about the man's riches--it's about the man's idolatry of his riches, and his need to come follow Jesus to have eternal life. Reading all of this together, it would appear that the message is that the only way to righteousness/justification is through coming to and following Jesus. He, and He alone, is the way to life.
--Second, note that Paul said this same thing: "The righteousness of God has been manifested...the righteousness of God through faith in Jesus Christ for all who believe" (Rom 3:21-22). He then continues on, talking about the cross. Referring to this righteousness as that which is manifested "through faith...for all who believe" is to say that the faith/belief the Christian has taps INTO this righteousness somehow. Note Paul's words in 2 Cor 5:21: "He (God) made Him to be sin who knew no sin (Jesus), that in him we might become the righteousness of God". Christians, with their sin placed on Jesus, have His righteousness placed on them. The two phrases the theologians use are substitionary atonement--that Jesus is our substitute, and imputed righteousness--that God accounts Jesus' righteousness to us who have none. Finally, note Paul's words in Philippians 3:8-9: "I count everything as loss because of the surpassing worth of knowing Christ Jesus...I have suffered the loss of all things...not having a righteousness of my own which comes from the law, but that which comes through faith in Christ, the righteousness from God that depends on faith." Truly if Paul COULD have "righteousness of his own from the Law", it would not be enough, for "by works of the Law, none will be justified" since the Law only shows us how we can't keep it (Rom 3:20).
The righteousness of God is given to those who have faith in Jesus' work. And the Gospel comes to us in the message of the cross. When one, empowered by the Holy Spirit (1 Cor 2:4-5), is brought to the realization of their dependence on Christ's perfect work on the cross--and the cross is only what it is because of Jesus' perfect life of holiness--they've been given new life, exposed to the truth, and raised with Christ into new life. They are not depending on their self to be acceptable to God--rather, they're depending on God's mercy in Christ's work to be acceptable to Him. What a wonderful truth!--that I can know God because Jesus has done all the work which I couldn't do!
Belief in Jesus and obedience to Jesus are synonymous in the New Testament--see Jn 3:36, Ac 6:7, Rom 1:5, and Heb 3:18-19. To obey Him, one must trust that He's worthy of obeying, and it is His cross, resurrection, and effectual calling which proves His worth. Righteousness before God, this side of the Fall, is something that is only revealed and received through faith in Christ. He is either all in all, or He is none at all. Let those who have been humbled by the Gospel cry out "the former!", and let those who have been humbled by man's depravity cry out "I believe, help my unbelief!". The former will treasure Christ as the all in all, and they will "go and make disciples"--but not by their own power. Rather, it'll be by Jesus' power--and His power comes to us through His Spirit. The latter will cry out to Him for wisdom, sure that He will answer (see 1 Cor 1:30 with James 1:5), because He gives rest to those who come to Him. "Surely I will never cast them out" (Jn 6:39 with Mt 11:28-30).