“So let me make sure I understand this. If someone totally rejects and repudiates their Christianity, they are just showing that they were never actually saved in the first place?”
That’s the doctrine, yes.
“Can anyone know for sure that they will never repudiate their Christianity (or more properly, Christ?)”
The Bible seems to be replete with reassurances on this. For example, “Let not your heart be troubled; you believe in God, believe also in Me. In My Father’s house are many mansions; if it were not so, I would have told you. I go to prepare a place for you. And if I go and prepare a place for you, I will come again and receive you to Myself; that where I am, there you may be also.” (John 14:1-3).
Also the examples of Job, and Peter, Satan is allowed to do so much, and no more. Throughout Biblical history, God keeps his people.
“At the least the teaching that a person can reject his salvation still allows for present assurance based on present faith”
That may be so; but it should be of concern to us to think Biblically, and accept what the Bible says, not be pragmatic about it.