It is said by some that when a Christian speaks of being saved, and especially when they speak of being sure that they are saved, they're expressing a kind of religious arrogance. After all, only God is privy to such things. Not at all unlikely, given our sinful nature. A game question.
Lip service is no service at all.
Modern humans have learned how to con ourselves and each other in ever better ways. Our ability to 'know' anything for sure is quite limited. Many Christians forget how much we material beings depend on trust, and lapse into talk of 'certainty' and 'absolute knowledge'. We do not have unqualified 'certainty' about anything. And, it comes by trusting that there has been a lot of checking and cross-checking over the course of 4000 or so years of Jewish and Christian history, to see if we understand God's communication rightly.