Wa`alaykum as-Salamu wa Rahmatullahi wa Barakatuh.
In the Name of Allah, Most Gracious, Most Merciful.
All praise and thanks are due to Allah, and peace and blessings be upon His Messenger.
Brother, we really appreciate your forwarding this question to us, and we commend your keenness on getting yourself well-acquainted with the teachings of Islam. May Allah help us all keep firm on the Right Path, amen!
There is no doubt that faith increases and reaches very high levels at times and decreases at other times. When faith decreases, it is important for a Muslim not to deviate from the right path. We can do this by keeping firm on doing at least the obligations of Shari`ah and not commit major sins.
At the times when a Muslim’s faith is weak he should hold himself accountable for his deeds and ask himself why he got in this state. He should do his best to increase his faith by seeking the help of righteous friends, reciting Qur’an and making more dhikr (remembrance of Allah) and asking Allah the Almighty for forgiveness.
In his response to your question, the prominent Muslim scholar, Sheikh Muhammad Iqbal Nadvi, Director and Imam of Al-Falah Islamic Center, Oakville, Ontario, Canada, states,
This is normal. You cannot maintain the peak moments of worship all the time as everybody goes through times of weakness and lassitude. What you need to do is to be sure you do not deviate from the right path when slackening or leaving worship or going beyond the minimum requirement. For example, when you do not feel a strong desire and enthusiasm for salah (Prayer), you should continue to pray and keep the minimum requirement, meaning the fard (obligatory) salah, until you regain your energy and do a lot of voluntary salah.
Therefore, if at your weak points you are still committed and doing the minimum requirement, then you are still on the right path. However, if you start to even lose the minimum level of worship, for example, you neglect salah and do major sins, then you are deviating from the right path and you need to repent sincerely to Allah the Almighty and renew your commitment with Him.
Allah Almighty knows best.