Calvin on temporary, deep in the heart faith

This passage seems a little odd, given the Reformed doctrine that one can know that one will persevere, that one is elect. In this passage from Calvin's Institutes he states that even those who will fall away--i.e. they do not have the gift of perseverance, will have what is preached take deep root in their hearts:

There will be no ambiguity in it [Mt. 22:14], if we attend to what our former remarks ought to have made clear--viz. that there are two species of calling: for there is an universal call, by which God, through the external preaching of the word, invites all men alike, even those for whom he designs the call to be a savor of death, and the ground of a severer condemnation. Besides this there is a special call which, for the most part, God bestows on believers only, when by the internal illumination of the Spirit he causes the word preached to take deep root in their hearts. Sometimes, however, he communicates it also to those whom he enlightens only for a time, and whom afterwards, in just punishment for their ingratitude, he abandons and smites with greater blindness. (Calvin Institutes III 24.8)

It seems to me there can be no assurance at all if it is possible for God to enlighten us so that the word takes deep root, but then later he abandons and consignes to even deeper darkness due to ingratitude. Indeed, the passage above even implies this is God's plan. This goes far, far deeper than even the temporary faith I blogged about before because at least temporary faith was described as a sort of false faith. Here Calvin says one can have true faith for a time and yet have this gift taken away. He postulates two species of calling, but there can be no assurance that one is effectually called at any given time--God could remove what he has placed in one's heart just like he can implant it there. That this is just like the faith of the effectually called is brought out by his statement that "for the most part" God only grants deep faith in the heart on believers.

The more I read of Calvin, the more I see that what he purportedly gives for assurance he takes away due to implanting doubt in those who want to know they are elect. The pastoral difficulties are readily apparent: if Christ died only for the elect and if the faith deep in my heart today can be taken away tomorrow due to ingratitude, where is the assurance?