Alma is here returning to the question of signs. He has indicated that signs to not generate faith, but rather knowledge, and here he notes that knowledge can be a greater condemnation, should one fall away. Alma says that one who knows and transgresses is under a greater condemnation than one who only believes and transgresses. This is the meaning of his statement, but what is the purpose?
Alma is preaching to a people who are attached to an apostate community. They are coming to Alma because they have been shut out of what Alma understands to be an apostate religion. What Alma is doing is opening a door for them. They may not ask for signs, but they may begin to develop faith in God in the proper way. Alma is laying the groundwork for their ability to change. He tells them, in essence, that they are in the belief state, and that there is not as heavy a condemnation against them for their erroneous belief. They may open themselves, through their humility, to true faith.