When you talk about “100% sure,” you are asking me if I think something is the case. And I could answer that question in a multitude of different ways. I can say to you, “No.” I could say, “I don’t think so.” I could say, “Absolutely not.” Or I could say, “Yes, maybe,” “I hope so,” or, “I think so.” Or, I can say, “Absolutely certainly.” Those responses indicate various degrees of certainty that are associated with a particular question.
And when you ask me the question, “Can a person be 100% sure of their salvation?” I don't know how to break that down into percentage points. But we are certainly called to have a full assurance of our salvation by sacred Scripture. And I do think it’s possible to have what we would call an assurance—a full certainty—of whether we are in the kingdom or not in the kingdom.
The problem emerges, however, when there are people who are sure that they are saved, but they aren’t saved because they have a faulty understanding of what salvation is or a faulty understanding of where they are in their own faith.
But if we have a biblical understanding of what salvation is and an understanding of who we are, then I think we can rise to a very high level of assurance of our salvation.