This phrase ‘that you may know’ is found several times in the first letter of John. Some in John’s day claimed secret knowledge to separate themselves from others in the church. They seemed to be claiming ‘only we know…’ to draw others to themselves, but John writes, ‘that you may know’…
“These things I have written to you who believe in the name of the Son of God, that you may know that you have eternal life, and that you may continue to believe in the name of the Son of God.” 1 John 5:13