Two outlooks on Christian decision-making: In the first, one sees the good that might be done, and the abilities one has to realize that good. One acts to maximize the good. In the second, one looks at the corruption and blemish that one might incur by following different courses of action. One acts to minimize the bad.
How this might play out*: Parents are choosing where to send their bright and talented son to high school. They are considering a top-rated public school where their son will meet a racially, economically and intellectually diverse group of students, take challenging courses and have the opportunity to participate in a wide range of activities. There are not many Christian students or teachers at the school, and there is substantial drug use and promiscuity. Alternatively, they consider a small church-affiliated high school where their son will easily be a top student, all students and teachers will be white Christians and there will be few extracurricular opportunities. Parents with the first outlook would most likely focus on the benefits of the challenging environment and choose the public school. Parents with the second outlook would most likely see the potential bad influences of public school students and choose the Christian school.
Reasons why the first outlook is best: There is no precedent among the early apostles for the Christian isolationism that inevitably results from the second outlook; rather, their work carried great risk of physical and spiritual harm. The greatest moral commands are to love God and to love others, not to preserve one's own purity. Finally, the beauty and terror of creation and the bloody facts of the incarnation demonstrate that God himself will permit and suffer great evil in order to bring about great good.
Now, this does not mean that we should chase after every idea we have for doing good. It is, as unfortunate experience has taught me, foolish to date someone with the hope of evangelizing that person, or to attend anonymous lewd parties with the hope that one's mere Christian presence will impart a magical transformation on the place. We cannot calculate odds, but we must act with clear, important objects that have a reasonable chance of succeeding. The alternatives are foolish indulgence, and selfish fear of sins already cleansed.
*This scenario is fictional. The correct decision would obviously depend on the details of schools and people.