Astikyam means to believe in the injunction of the sastra, astikyam. That is called theism. One who does not believe in the injunction… Just like Krsna is advised in the Bhagavad-gita. One who believes in the words of Krsna, he is astika. One who does not believe, he is nastika. This is the astika and nastika. So our nastika definition means one who does not believe in the Vedic instruction. He is called nastika.

From Srila Prabhupada’s lecture on Bhagavad-gita 18.45 – October 11, 1975, Durban