We can know NOW that we have ETERNAL life. 1 John 5:13 says, "These things (the prior verses) have I written unto you that believe on the name of the Son of God; that ye may know that ye have eternal life, and that ye may believe on the name of the Son of God."
We don't have to wait until we are in heaven to KNOW that we are saved; we can KNOW it NOW, and I KNOW that I am saved. I'm sorry for any who don't have that assurance. However, that doesn't mean that just because some don't believe in eternal security and believe that we can lose our salvation, that doesn't mean that we have to go through life wondering and hoping that we haven't lost our salvation. If that were true, I would lose mine every day. God gives ETERNAL life--not temporary life until our next sin. We lose fellowship with God until we repent of that sin, but we DO NOT lose our salvation. That does NOT give us a license to sin since sin brings chastisement even to the point of death sometimes if we don't repent of any sin we commit AFTER we're saved. God knows we still have our sin nature, and that we will struggle with that until we're called home.
Just so that you don't think I'm living in deep sin and claiming to be a Christian, my only sin any more are my thoughts that are displeasing to God. I cannot claim that my thoughts are always pure, but I repent of those thoughts each time.