A Roman Catholic apologist recently posted this: "John Calvin: Only God Knows Who is Numbered Among the Elect":
“We can't know (any of us: Protestant or Catholic or green-eyed, one-legged, red-haired Rastafarian moth catcher) if we will be eschatologically saved because none of us know the future. You may claim to know that by some interior revelation or something, but you really don't.”
I don’t think I have ever met a Roman Catholic who claims to know with certainty he or she is saved. The Roman Catholic theologian Ludwig Ott has said, “The reason for the uncertainty of the state of grace lies in this: that without a special revelation nobody can with certainty of faith know whether or not he has fulfilled all the conditions which are necessary for achieving justification.”
Contrary to the above, 1 John 5:10-13 says:
1John 5:10 He that believeth on the Son of God hath the witness in him: he that believeth not God hath made him a liar; because he hath not believed in the witness that God hath borne concerning his Son.
1John 5:11 And the witness is this, that God gave unto us eternal life, and this life is in his Son.
1John 5:12 He that hath the Son hath the life; he that hath not the Son of God hath not the life.
1John 5:13 These things have I written unto you, that ye may know that ye have eternal life, even unto you that believe on the name of the Son of God.
John tells us he wrote that “you may know you have eternal life.”
A theologian of glory will usually then argue something like "how can you prove that you know?" In which case, I strongly recommend they pick up a copy of Bertrand Russell's book, The Problems of Philosophy"- since the question being asked is not the question of a theist, but that of an atheist. Russell demonstrates the slippery and elusive nature of all knowledge- from the perspective of atheism.
As Christians though, we begin with the presupposition that God has spoken, and what He says is true. God says I can know, so therefore I can. I have more to say on this over the weekend.