The allegory of belief [iman] is that of a land that has five walls. Al-Hajjawi said in its commentary: It is said that the allegory of belief is that of a land that has five walls. The first wall [the innermost wall] made from gold, the second from silver, the third from iron, the forth from cooked clay [aajurr], and the fifth [the outermost wall] from brick.
As long as the people of the brick wall are diligent in protecting the brick, the enemy does not aspire [destroying] the second; but if they neglect this [brick wall], they will aspire for the second and then the third, until they demolish all of the walls.
And like belief [iman] has five walls: certainty, then sincerity, then performing what is obligatory, then the recommended [sunan], and then refined behavior [aadab].
As long as one holds to and is diligent with having refined behavior, Satan does not scheme [to destroy] him. But if one forsakes refined behavior, Satan aspires to [destroying] the recommended works, then the obligatory, then sincerity, and then certainty.
[Source: Al-Saffarini, _Ghudha al-Albab_. 1:27, taken from Sunnipath.com]