In order to understand Molina's relevance for Kant with regard to grace, we need to understand Molina's distinction between prevenient and justificatory grace.

 For Molina, the outcome of a successful divine and human cooperation with regard to prevenient grace is a form of faith, a so-called 'faith of the heart' which in turn involves the right form of commitment. To repeat: neither prevenient grace nor human will-power in their own right can make this happen. Should the effect in question, i.e. faith of the right kind, occur, then – on the basis of it – justificatory grace, as a second stage or aspect of grace can set in or take effect, through which the person in question is taken as justified despite her original shortcoming. This latter issue of course involves many problems in their own right, and I am not claiming that the Molinist account – sketched so far – can provide a solution for them. Strikingly, it is the issue of justificatory grace which usually takes center stage in the literature on Kant, on account of its prominence in the pertinent sections of the second part of the Religion, but the passage in the guarantee footnote may indicate an important connection of justificatory and prevenient grace in Kant as well.

 In a rarely considered passage of Kant's Perpetual Peace Essay mentioning grace and faith, there are, first of all, two different dimensions involved, namely on the one hand faith in the sense of an act or disposition, and on the other the contents of this faith.
Given what has just been said about Molina's two-phase account of grace, this passage is perfectly consistent with such an idea. In this vein, it is faith as an act or disposition which requires something like prevenient grace, while the contents of this faith has to do with Kant's justificatory grace. Read this way, the question is then whether the "deficiency of our own justice" refers to the situation prior or posterior to prevenient grace. It would seem as though the formulation is flexible enough to allow both options, and the plural in "through means incomprehensible to us" may even be indicative a multi-phased provision of grace.