I say Hogwash!
Then you are calling the apostle John and God liars...
1Jn_5:13 These things 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.
If there was more for salvation John couldn't have been telling the truth...But you can bet your life that he was...
There is one church father who knew an apostle personally...That was PolyCarp...And he was as Protestant as I am...
Ok yes, we can know that we have eternal life, great. It’s not exactly clear what kind of eternal life. One with the Lord God creator of the Universe or one of eternal separation and torment, how do you know that?