The following is a post on christianforums.com that is a perfect example of how Once Saved Always Saved adherents view people who have left the religion of Christianity:
You use the phrase “was a Christian”.
Christianity is not something you can “put on today and take off tomorrow.” It is not a fad that you get over. Some churches will teach a person can be saved by the grace of God, and become so full of sin afterwards that they lose that salvation, and others will teach that once you are saved by Christ nothing can snatch you from the hands of Christ, but all of them will tell you that Christianity is not something that a person just “decides to be today” and “decides not to be tomorrow.”
They will all confirm that once you have met the living Christ, you can NEVER deny that Christ is Christ.
So to many Christians, you would not be a “former Christian”. You would be a person who went through the motions of Christianity without having ever really known the Savior.
There are a lot of people like that in churches. They grew up in churches, they learned the lingo, but they never really committed their lives and hearts totally to Christ so they were never really Christians.
They may never realize just how offensive this truly is.
I see this as a very clear explanation of how Once Saved, Always Saved adherents view ‘former Christians’.
This could quickly become a “No True Scotsman” discussion, which is often the case when the topic of OSAS comes up (which is what the above view illustrates).
Saying no “real Christian” would leave Christ, deny him, or leave Christianity is, in my view, a logical fallacy (see link above). In order to explain the salvation conundrum of those who leave the religion of Christianity, one has to assume they are either still saved and just don’t recognize it, or they were never saved in the first place (or “they never met Jesus”, or “they just went through the motions”).
Those are the only ways I know of to explain someone leaving Christianity from the OSAS perspective.
Many people leave Christianity because of a reasonable decision (not because they are full of sin, as stated in the above post). The judgment that these people must never have known Jesus personally or that they just went through the motions is insulting and presumptuous. No one knows what their lives were like. No one has the insight on whether they “truly met Jesus” or not. To say such a thing (as is often said when the OSAS argument is vocalized) is offensive and judgmental.
Clearly I disagree with the OSAS theology in general, and always have, but this is one reason why. It requires these sorts of No True Scotsman fallacies from those who believe it.