The Catholic Faith is a series of beliefs and practices. The Catholic Church is a specific institution to which people belong. Some people who accept most, or all, Catholic beliefs, and even may identify themselves as Catholic, are in other churches; for instance, the Continuum or LCMS. Some Eastern Orthodox identify themselves as Catholics.
G. K. Chesterton probably believed most or all Catholic belief for many years before he became Catholic, probably because his wife was Anglican, and not ready to swim the Tiber. (Both eventually did). I had a few friends who left the Catholic Church. About once a year.
But when I would talk to them, they still hold most of the assumptions of Catholicism, perhaps without realizing it.
Many would go to Pentecostal church services half the time, and RC services half the time.
I have known others who have - mostly - left the Catholic Faith, but not the Church. They may hold employment in Catholic institutions. Or even if they are volunteers in some ministry, they may regard themselves as kind of missionaries - sort of using their position to get the Church to change its position, from the inside; often to support women’s ordination, Peace and Justice, etc.
This was a much bigger issue a few decades ago than now, as many of that generation have passed away. Young people who have radical or even very liberal mindset don’t even enter the convent or religious life today.
Again, it makes a difference whether Catholics leave the Catholic Faith (or Catholic Church) to go Protestant, or to go secular. Sometimes it is gradual, sometimes it can be some of both Protestant and secular. I knew a few who feel the “need” to worship somewhere on Sunday, even if they don’t particularly identify with the Protestant or Catholic church they happen to visit. There’s the music, the religious setting, some connection here with other people. Nothing to do with theology.
They may even bring their kids to Christmas at church, or even a little Sunday School as long as it is liberal and non specific, because “kids need something like religion growing up”. In his autobiography C. S. Lewis described his tutor who had left behind all vestiges of his Christian, Presbyterian upbringing, except that he always dressed a little formally on Sunday.
