Praise be to Allah, and peace and blessings be upon the Messenger of Allah. To proceed:
Weakness of faith (Iman) is something that may affect a believer from time to time, but it is not a permanent condition. Allah has provided many means to treat it and strengthen one's faith.
Among the most common causes of weak faith are:
Among the greatest means of strengthening faith are:
Allah, the Exalted, says:
"And We send down in the Qur'an that which is a healing and a mercy for the believers."
He also says:
"And every account We relate to you from the news of the messengers is that by which We make your heart firm."
Weakness of faith may affect a Muslim, but it is treated by turning sincerely to Allah, increasing remembrance and seeking forgiveness, reciting the Qur'an with reflection, remaining steadfast in acts of worship, avoiding sins, and seeking beneficial knowledge. The closer a servant draws to their Lord, the stronger, more steadfast, and more tranquil their heart becomes.
And Allah knows best.