"leaving the faith"... is not losing salvation, as if you gave God your faith, one time, then God gave you His Salvation, that is eternal.
However, the UNDERSTANDING regarding how to understand and exist in REAL FAITH, can become "darkened", in a born again believer.
Paul talks about this in Galatians 3, as they left the real faith...= the UNDERSTANDING of the Gospel of the Grace of God.
Paul said '"you started in the Spirit and now you try to continue in the Flesh".
That means, they once understood that Jesus kept them saved, that Jesus Himself IS their Salvation... but they lost that understanding..."fallen from Grace".... and now they are trying to stay saved and go to heaven, based on "flesh", = Works, commandment keeping, water baptism, ... anything but simple TRUST in CHRIST..= to keep them saved.
They do not believe that Jesus keeps them saved. They can't any longer as their mind has become darkened by deception. "bewitched", as Paul describes it.
This believer has lost the understanding that is the "simplicity that is found IN CHRIST".. and are now trying to not lose their salvation, by self effort.
= Legalism.
If you talk to a person who is "fallen from Grace, "deceived", in the flesh"...>"bewitched", They will say....."as long as i DO THIS,, and as long as i DONT Do THAT..... i will go to heaven".
See the "Do's and Donts'? That is their SAVIOR, and that is Not JESUS.
That is not real faith in Christ, that is SELF RIGHTEOUSNESS, ....= trying to make self RIGHT with God... and that faith is broken....."fallen FROM Grace".
Salvation is God's Righteousness, given as "the Gift of Righteousness" that is our Eternal Salvation.
God's Salvation is Christ on the Cross, and once this is applied to you, its DONE.
However, its very easy to lose this knowledge and get swept away from this Real faith, into Self SAVING.
A person or a denomination or a cult, will cause you to lose the right understanding of Salvation that is a GIFT from God.
When they do this to you, Pauls says...>"who has BEWITCHED YOU< that you no longer OBEY the Truth".
Who will do this to you?
A.) Forums are filled with these people.
B.) Pulpits are filled with these people.
Its Anyone who believes they can lose their salvation, as that person, is "fallen from Grace" and cannot teach the True Gospel, as they have left it, they are "bewitched", and they will do the same to you, and have...if you believe you can lose your Salvation.