Because of your unbelief: for verily I say unto you, If ye have faith as a grain of mustard seed, ye shall say unto this mountain, Remove hence to yonder place; and it shall remove; and nothing shall be impossible unto you. Matthew 17:20
Faith is the key to achieve what we need. Without this, it will not be possible, since we are strengthened through it to fight for the impossible and overcome the obstacles that arise in our everyday life.
Faith is not to be resolute today and have a change of mind tomorrow; faith is to stand our ground from beginning to end, not wavering for a single moment. That is why Jesus made the comparison with the grain of mustard seed for them, so that they would see the result of faith.
Faith grows every day; that is why we need to exercise it. It is not the same faith that we had when we arrived at the feet of the Lord as we have now, since it has strengthened through all trials and battles we have had to endure.
There is no excuse to say “I don’t have much faith to fight this battle,” for you only need it to be the size of the grain of mustard seed. Then you will say “Remove!” to the mountain, and it will be removed.