Not to beat a dead horse with the gospel and social justice conversation, but when it comes to social justice as implication of the gospel rather than content, related to my objection to piggybacking something we do into the good news of what Christ has done is the salvific neutrality of morality.
Here's what I mean by that: In my current ministry context it has become important to rehabilitate the perception of the Church's message because most lost people (and plenty Christians) in my area believe the message of Christianity is "do this" or "don't do that." The message of the gospel is thought to be "Behave!" Most have rightly rejected this message, reasoning quite logically that they can be "good" apart from Christ and his Church, and in fact many are.
Heck, Angelina Jolie has adopted, what?, 300 orphans? Isn't George Clooney saving the world? Matt Damon fights for social justice, and he does it without a splinter's notion of the gospel.
Are we supposed to be doing these things too? Yes. It is the command of God to love our neighbor.
But social justice is not unique to Christianity, and in fact if social justice is the good news, we bear the same message as lots of people who are going to hell. Social justice, then, is salvifically neutral. And therefore, while it accompanies and may testify to the good news, it cannot be the good news itself.
The charge against the "riches of Christ" gospel is that it is too simplistic. As if the raising of a dead heart and power for discipleship is small!
No, what Christ has done to forgive, raise, and reconcile sinners to God is huge. It is all the complexity of God and man simultaneously existing in one Person, and his sinless life, sacrificial death, and bodily resurrection empowering conversion and obedience. We may reduce the gospel's message -- as Paul does in 1 Corinthians 15:1-4 -- to this bare historic fact and still have something of eternal interest to gaze-happy angels.