What does the Bible say about undoubting faith?

In Matthew 21:21, Jesus states that if one has faith and does not doubt, they can not only perform miracles like cursing a fig tree but also command mountains to be cast into the sea. This captures the essence of undoubting faith, which is a complete and unwavering trust in God's power and promises. It acknowledges our weakness and the struggle of faith from our old nature while affirming that true faith is given by God, enabling us to believe without hesitation in His abilities and intentions.