Signs do not generate faith but rather knowledge. Knowledge can be a greater condemnation, if one should know, but then transgress. The person who only believes and transgresses has less responsibility. Why does he make this point?
Alma’s listeners are attached to, but excluded from, an apostate community. Alma is laying the groundwork for their ability to develop faith in Yahweh in the proper way. He tells them, in essence, that, although they may have espoused erroneous beliefs, they are just in the early stages of belief. There is not as heavy a condemnation against them, especially since their humility opens the possibility to develop true faith. They are yet able to repent.