“Faith as small as a mustard seed” is an extremely misinterpreted verse.

The context of this verse in Matthew is Jesus’ followers wondering why they could not drive out a demon, to which Jesus responded “because of your little faith…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”

This verse is not meant to reassure people with little faith, it was to show how even his followers didn’t have faith as small as a mustard seed, because they couldn’t drive out a demon, let alone move mountains.