Matthew 17

To the Savior -

"...there came to him a certain man, kneeling down to him, and saying,
15 Lord, have mercy on my son: for he is lunatick, and sore vexed: for ofttimes he falleth into the fire, and oft into the water." (New Testament | Matthew 17:14 - 15)

What is interesting is that the man confessed -

"...I brought him to thy disciples, and they could not cure him." (New Testament | Matthew 17:16)

The Savior then healed the man's son. Afterwards -

"...came the disciples to Jesus apart, and said, Why could not we cast him out?
20 And Jesus said unto them, 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.
21 Howbeit this kind goeth not out but by prayer and fasting." (New Testament | Matthew 17:19 - 21)

This experience suggests that some acts require more faith than others. I also believe that individuals possess varying degrees of faith in differing matters. And that is some cases, sufficient faith requires prayers and fasting. Elder James E. Talmage wrote -

"The Savior's statement concerning the evil spirit that the apostles were unable to subdue—"Howbeit this kind goeth not out but by prayer and fasting"—indicates gradation in the malignity and evil power of demons, and gradation also in the results of varying degrees of faith. The apostles who failed on the occasion referred to had been able to cast out demons at other times. Fasting, when practised in prudence, and genuine prayer are conducive to the development of faith with its accompanying power for good. Individual application of this principle may be made with profit. Have you some besetting weakness, some sinful indulgence that you have vainly tried to overcome? Like the malignant demon that Christ rebuked in the boy, your sin may be of a kind that goeth out only through prayer and fasting." (JTC, p. 395