Saying that you are sure of anything, when there is a conflict in your mind about it, is being untruthful. But some Christian leaders preach that we ought to be sure in our opinions on certain subjects -- that it is a requirement of being a good Christian, and possibly of salvation.
It is a misunderstanding of God to believe that God might ever want you to be untruthful to yourself, much less that God might require it for salvation. It is one of the telltale signs that the believer is attributing a single perspective to the Omniscient One. A single perspective of consciousness is the attribute of a human ego. While human consciousness, and God's consciousness are the same in fundamental nature, a human ego is not a characteristic of God's consciousness. God "owns" all the egos of all the humans there are. But God does not have one human ego, unless we are talking about Jesus' ego (if He truly had an ego.)
The kind of salvation hoped for by a person who confuses God with someone having an ego, is perpetuation of the ego. The ego is not the source of happiness or fulfillment, so eventually, all people will understand that the ego is not what needs to be (and is) preserved. The ego definitely dies, wholly and completely. But that is only scary if you believe that the consciousness you know as yourself will die.
It is utterly critical, in order to get out of the basic fear of death that religion addresses, to:
1.) Acknowledge and accept that every experience and memory you have may be lost to you at some point.
2.) Come to know your own consciousness as a real, existing something, that is not just equal to all the experiences and memory that may be lost.
These might also be stated:
2.) Faith or belief.
When you have those two, you become aware of the gift of eternal life that you already have. The fact of that gift, and the fact that it can never be taken away from you -- because it is eternal, and it IS you -- are the Good News, that seems so hard for people to get.
To say it with chutzpah, if you don't understand this, you don't understand what Jesus said.