The whole man becomes defiled when he sins. The gnostic heretics thought that their behavior could never defile their spirit.
Irenaeus, Against Heresies
2. ... But as to themselves, [the gnostic heretics] hold that they shall be entirely and undoubtedly saved, not by means of conduct, but because they are spiritual by nature. For, just as it is impossible that material substance should partake of salvation (since, indeed, they maintain that it is incapable of receiving it), so again it is impossible that spiritual substance (by which they mean themselves) should ever come under the power of corruption, whatever the sort of actions in which they indulged. For even as gold, when submersed in filth, loses not on that account its beauty, but retains its own native qualities, the filth having no power to injure the gold, so they affirm that they cannot in any measure suffer hurt, or lose their spiritual substance, whatever the material actions [i.e., conduct] in which they may be involved.