Effort vs. Earning
Imagine that I told you I had paid for you to spend next summer in France. But it wouldn't be in Paris where there are lots of tourists; you would be living in the French countryside, tending a vineyard, surrounded by people who only speak French. What would you do to prepare for that trip next summer? You would learn French and learn a bit about vineyard work.
Peter tells us in 2 Pet. 3:13-14 that since we are "looking forward" to new heaven and new earth, we need to "make every effort to be found spotless, blameless and at peace with Him." Because we are in Christ, we now share in an inheritance: a new heaven and a new earth. And since we are now heirs with Christ, looking ahead to this new land, we prepare for it now. We start living the way things will be in advance of it. Since this analogy is an extended version of something N. T. Wright uses in "After You Believe", I might as well use his words. We are trying to live in the present "in the light of God's in-breaking future."
Such living requires effort. But effort is not the same thing as earning. Earning– in our France analogy– would be like me saying, "Look, if you learn French I'll pay for you to spend next summer in France." Effort is me saying, "Since I've paid for you to go to France, you'd better start preparing to learn the language." Of course, the analogy breaks down because for the Christian, Christ is at work in us– the hope of glory!– it is the Holy Spirit who helps us "learn the language of virtue". And every "yes" we say to Him forms the habit of living in the Spirit until it becomes second nature.