So the Lord said, “If you have faith as a mustard seed, you can say to this mulberry tree, ‘Be pulled up by the roots and be planted in the sea,’ and it would obey you." Luke 17:6
So Jesus said to them, "Because of your unbelief; for assuredly, I say to you, if you have faith as a mustard seed, you will say to this mountain, 'Move from here to there,' and it will move; and nothing will be impossible for you. Matthew 17:20
These verses make sense once we understand that they are only true when our will is according to God’s will. Whatever God's will is, that's the key, if we have even just the smallest amount of faith in Jesus, a tiny amount like the tiny mustard seed, God can and will still accomplish His will, no matter how large or impossible. For us to come to know His will is impossible in the flesh, but it can be spiritually discerned through our faith in Christ and His Word. But Christians, even with the weakest of faith, can accomplish the will of God no matter how miraculous because “all things are possible with God.”
These verses are about who we put our faith in, not our faith and how much of it we have. It’s about our great God, not us! It’s not about our ability but our faith in Jesus and His ability. Seek His will and even mountains will move, in accordance with His will. When our will lines up with His will, our little mustard seed will move a mulberry tree.