Question: Can those who do not believe in Bodhisattvas be saved by them?
Answer: The very definition of Bodhisattvas is that they are those who aspire to guide all beings to Buddhahood, which is the ultimate salvation, liberation from all suffering, and attainment of True Happiness. In this sense, yes indeed, all will be saved by them, be one a non-believer in them or not.
However, for Bodhisattvas to be able to help a sentient being, that being must have some karmic affinity with them too, be it from a past life or this present life. Without karmic connection, that being cannot be helped. Such are the natural workings of the law of karma.
That said, it is difficult to tell if one has affinity with a Bodhisattva or not. The good news is that Bodhisattvas, in their ongoing noble, compassionate and equanimous quest to help all beings, will keep doing their best to create affinities with all beings, including non-believers. Of course, taking two hands to clap, it helps that the beings to be helped take an interest in learning about them – which creates affinity directly.
