Faith.

Faith is NOT something which turns on and off. Hence the oft presented statement about some Christian 'who has lost his/her faith' is an absolute non-starter.
In such circumstances all we are talking about is a mere opinion swapped for a different one. That has had little to do with faith.

Faith is NOT straining in a manner similar to constipation in order to get your belief system 'right' or straining in a similar manner to have a prayer answered.

'Saving faith' is an entirely different matter. This is a decision to repent and follow Christ which brings about your eternal salvation. [Arguments rage amongst believers as to whether somebody can then reject this position or not.]

Getting to a position of salvation is one thing, having the faith needed to be able to 'live the life' and 'have prayers answered' is quite something else.

Consider the story of the man with the demon possessed son. Jesus was angered when he doubted whether or not Jesus could heal the boy.
The father sensibly said. "I do believe - help me overcome my unbelief." This is the place where so many Christians find themselves on a daily basis. They know they have the salvation in place but find it difficult to 'live the life' successfully. This applies to ALL believers, I reckon.

Consider Hebrews 11 v 1: "Now faith is the substance of things hoped for, the evidence of things not seen."
This does not happen automatically but requires a 'close walk with God'. That needs work: study of His Word; prayer; praise; worship; obedience; thanks; LISTENING!
There has to be an ongoing attempt to 'clean up our act' and full repentance when we slip - as we inevitably do!