Yes faith can be as small as a mustard seed. And a mustard seed is very small. The key is that it is faith that we are speaking about. Without believing in God what is there? God is the creator. He designed everything that we know. Now there are some who question all aspects of creation. All I can say is Oh well. Faith in God and Jesus is so crucial. Again to many it is not. I cannot speak for them. All I do know is that the Bible was written about times over 3000 years ago. And man has not changed much since.
Matthew 17:20 He replied, "Because you have so little faith. I tell you the truth, if
you have faith as small as a mustard seed, you can say to this mountain,
'Move from here to there' and it will move. Nothing will be impossible