There are others who believe that once a person gets saved they can lose their salvation over and over again. I do not belong to either of these camps. When we enter into the blood covenant with Jesus Christ, we enter a marriage. Jesus is the Bridegroom and the church is His bride. Jesus will not leave us or forsake us, but He won't make us stay either.
People can, and do, leave their marriage partner for some whoremonger on the side.
This is born out in the spiritual realm also. I do not believe that this happens often. That is why this man's email is so strange. God is holy. We need to really think about that.