Which

Hello...

In the underlined part below, the subject of the sentence “it” refers to “the thought that cures”

Grammatically, the “which” can refer to “it”, “an appeal” and also “truth”?

Are three different interpretations possible grammatically, if further context is not given?

Actually, “it” and “an appeal” seem to be the same, so can there be two different interpretations?

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This is no magic. It is merely an appeal to truth, which cannot fail to heal and heal forever. It is not a thought which judges an illusion by its size, its seeming gravity, or anything that is related to the form it takes. It merely focuses on what it is, and knows that no illusion can be real.