[KitJ]

"So “be the mustard seed” can be interpreted to mean start small and you can still accomplish great things OR it could mean if you start with a small amount of faith it will grow to be a deeper faith over time."

Faith can move mountains. Even the tiniest speck of true faith. Small as a mustard seed.

[AZLiberty]

Faith can move mountains. Even the tiniest speck of true faith. Small as a mustard seed.

Yours is an accurate summary. Here's the relevant passage:

Matthew 17:18-20 -- 18 Then Jesus rebuked the demon, and it came out of the boy, and he was healed from that moment. 19 Afterward the disciples came to Jesus privately and asked, “Why couldn’t we drive it out?” 20 “Because you have so little faith,” He answered. “For truly I tell you, if you have faith the size of a mustard seed, you can say to this mountain, ‘Move from here to there,’ and it will move. Nothing will be impossible for you.”

Jesus was giving the disciples a rough time for not even having faith the size of a mustard seed.