Corrosion occurs when metals are gradually destroyed by chemical and/or electrochemical reactions with their environment. Perhaps the most common form is rusting, the formation of iron oxides, which results in a distinctive orange colour on the normally silver-coloured metal, typically caused when iron is corroded by water or air moisture.
Given sufficient time, oxygen and water, any iron mass will eventually rust to the point of distintegration, as the image above demonstrates. Clearly, such a problem has led to many working to prevent corrosion (by creating, for example, rust-resistant alloys such as stainless steel or by galvanisation, protecting the metal with a layer of zinc.)
In the spiritual realm, corrosion is just as destructive to spiritual health as rust is in the natural world. It is not caused by exposure to water or oxygen, however; it is caused by bitterness and resentment, by holding on to a grudge, and by an unwillingness to forgive others.
Forgiveness is at the heart of the gospel, and Jesus calls us to pass on all we have received from Him freely. The command to forgive is unequivocal and without limits (see, for example, Matt 6:12, 14-15; Matt 18:21-35; Mk 11:25; Luke 17:3-4; Eph 4:32; Col 3:13) and is the only known cure for spiritual corrosion. Without forgiveness, we hold on to bitter roots which destroy us and defile others (Heb 12:15). Only as we forgive do we find freedom from anger, bitterness and resentment, and the joy and peace which God promises to His children.