Jesus’ response to his disciples here was akin to saying, “who told you your faith needs an increase? If you truly have faith it does not need an increase because it was never decreased other than by religion and ignorance.”
So, instead of giving a formula for increasing faith, the Lord summarily ‘decreased’ their faith by saying “If you have faith”. The Lord was teaching a very powerful principle here. He was saying there is no correlation between the size of your faith and what it can accomplish. A faith as small as a mustard seed can pull up a mulberry tree by its roots and plant it in the sea.
When you feel your faith is little, remember the operation of the mustard seed. It does not thrive based on its size, rather it understands that what matters is its nature.