Matthew 13:10-13 ESV
10 Then the disciples came and said to him, “Why do you speak to them in parables?” 11 And he answered them, “To you it has been given to know the secrets of the kingdom of heaven, but to them it has not been given. 12 For to the one who has, more will be given, and he will have an abundance, but from the one who has not, even what he has will be taken away. 13 This is why I speak to them in parables, because seeing they do not see, and hearing they do not hear, nor do they understand.
Until the Spirit opens their eyes and ears, no one sees or hears. That is done when they believe and are given the Spirit. Having the Spirit they can receive more. Without the Spirit they receive nothing.