Faith of a believer need not always maintain the same level. As the days go by people grow weaker in their faith. This is true of faith as trust, our ability to trust God but also of faith that controls our moral behavior. Faith in Christ has made us new persons, people with a new value system, a new world-view etc. However, this may deteriorate as the days go by. Loss of sensitivity to sin, casual approach spiritual disciplines, general coldness to spiritual matters, lack of love for God and other believers are some of the symptoms of this. This is caused by the overwhelming influence of the world that we live in from which we have made a departure. The kingdom of darkness (the old way of life, values and world-view) hasn't given up the fight and always has been trying to take back the grounds it lost. It is possible that believers of Christ succumb to this. Peter calls this " the corruption that is in the world caused by evil desires" (2 Peter 1:4).
Analogy with iron probably may help us to understand this concept. Iron exposed to the elements rusts. When exposed to moist air, a piece of iron forms an oxide which degrades the metal slowly and finally makes it useless as iron. Christians are in the world, susceptible to its temptations, which corrupt the moral dimensions of their faith. Finally, they become one with the world. However, stainless steel never rusts! When 11% Chromium by weight is added to steel to make stainless steel it resists the formation of the oxide and thus rusting. It can withstand corruption.
Peter continues by suggesting how to fight corruption. The same metaphor from metallurgy helps us to understand this. He encourages his readers to "add to" their "faith" (2 Peter 1:5) seven virtues that will fortify their faith just as steel is fortified by Chromium. These virtues are: goodness, knowledge, self-control, perseverance, godliness, mutual affection and love.
Faith as a tool to get things done by God or faith as an identity tag that connects us with a community rusts and turns useless unless it is fortified by Christian virtues. For example, a Christian without self-control would turn to a carnal being very fast. Or a Christians without love for each other puts up a bad show before the world.
Rusting can be prevented by giving the iron a coat of paint! However, the paint also peels off when the elements are harsh on it. Some make futile attempts to protect their faith by a thin coating of outward spirituality. It doesn't help. Iron can also be protected by keeping it away from air and moisture, but such a piece of iron is useless! People have tried to keep their faith uncorrupt through asceticism, by keeping themselves away from the corrupting world. However, taking life taken out off active engagement with the world is useless too. The only option seems to turn our faith to stainless faith, the faith that can stand the world by fortifying it with Christian virtues.