I have read that it is the sin of presumption to say that you know you are going to heaven, yet at the end of Paul’s life he wrote, “From now on the crown of righteousness awaits me” (2 Tim. 4:8) and John said, “I write these things to you so that you may know that you have eternal life” (1 John 5:13). Were John and Paul committing sin when saying that they know they have eternal life?
While we do not have an absolute assurance of salvation, we do have a moral assurance. That is, we are assured of salvation provided we die in a state of grace—free of unforgiven mortal sin. With this in mind we can understand the confidence with which Paul and John write.
Presumption is different. It presumes salvation may be attained either (1) through one’s own abilities, or (2) through God without one’s own cooperation. The Catechism of the Catholic Church explains,
There are two kinds of presumption. Either man presumes upon his own capacities, (hoping to be able to save himself without help from on high), or he presumes upon God’s almighty power or his mercy (hoping to obtain his forgiveness without conversion and glory without merit). (CCC 2092)