This can be compared to a winter frost. If the ice on a river is thick at first, and then thins out, one can still cross the river, for the ice remains very strong. However, if we see that the ice is thin and weak, it is a sign that it was never [thick and] strong to begin with. Likewise, if we see a person who serves G‑d only occasionally, it is certain that he has never yet served Him properly, for had he served G‑d properly even once, he would serve Him always. *Ibid.*p.14a