Two views what it means to have faith like a grain of mustard seed:
At times, faith in Jesus seems like it can be dismissed as being as small as a mustard seed, but with this humble faith, mountains can be moved. The question is thus not whether one has only a little faith, or whether one’s faith is large enough, but whether one truly has any genuine faith. God grants to those with such faith the new heaven and the new earth. With such a faith even prayer that seems to break the natural laws of our present world may be effective. The prayers of the saints shape the world into a new creation (Revelation 8:3–5). Faith in Jesus’ name overcomes the world (1 John 5:5). Nothing is impossible for the one who believes. Paul could do all things through Christ who strengthened him (Philippians 4:13). In Matthew 17:20, Jesus speaks of active faith, albeit faith as small as a mustard seed—so small as to be virtually invisible and looking like unbelief. It is neither faith's amount, nor its size, that matters, but the greatness of the One in whom faith is placed: the omnipotence of God and of Jesus. Even faith no larger than a mustard seed (the smallest seed visible to the human eye) can move mountains: can make the impossible possible. Such faith involves total surrender to God and submission to Jesus as his disciple.
The image of a mustard seed recalls that image used in Matthew 13:31, explaining why faith, which must lead to action, is on the same level as a seed, which in itself does not yet act. Although a seed is a small germ, it has the potential of growth within it, just like faith. The apostles’ faith may yet be small, but it will grow. As well, Jesus puts a task in front of faith, even when still in its germinating stage. That task can be done only through faith’s life-power, which God causes to grow, so that it can achieve what it must.
Interpretation 1:
At times, faith in Jesus appears insignificant and is easily dismissed. It is as small as a mustard seed. Yet when one possesses this humble faith, mountains can be moved. The decisive question is therefore not whether one has only a little faith or whether one’s faith is large enough, but whether one truly has any genuine faith at all, even if it seems small. Though such faith may be overlooked by people, God grants to those who possess it the new heaven and the new earth. With such a faith even the prayer that appears to contradict the natural foundations of our present human existence may be effective. The prayers of the saints reshape the world into a new creation (Revelation 8:3–5). Faith in the name of Jesus overcomes the world (1 John 5:5). Nothing is impossible for the one who believes. Paul could do all things through Christ who strengthened him (Philippians 4:13).1
In Matthew 17:20 Jesus speaks of an active faith, albeit a little faith—a faith as small as a mustard seed, which is so small as to be virtually invisible and effectively identical to unbelief. It is not the amount of faith or the size of the faith that is ultimately decisive, however, but rather the greatness of the One in whom faith is placed: the omnipotence of God and of Jesus. Even if faith is no larger than a mustard seed (which is the smallest seed visible to the human eye), it can still move mountains—that is, make the impossible possible. Such faith always involves complete surrender to God and a relationship to Jesus that is characterized by discipleship.2
Interpretation 2:
This image of a mustard seed recalls the same image used earlier in Matthew 13:31. That would explain why faith, which absolutely must lead to action, is placed on the same level as a seed, which in and of itself does not yet act. Even though the seed is a small germ, it contains the potential of growth within it—and this is exactly how faith works, which is comforting. The apostles’ faith may still be small now, but it will grow; it cannot remain at rest. A second point that stands out is that Jesus here places in front of faith, even when it is still in its germinating stage, a task. That task can be fulfilled precisely because faith has life-power within itself and God himself causes it to grow, so that it can accomplish what it must.3
20 C'est à cause de votre incrédulité, leur dit Jésus. Je vous le dis en vérité, si vous aviez de la foi comme un grain de sénevé, vous diriez à cette montagne: Transporte-toi d'ici là, et elle se transporterait; rien ne vous serait impossible.