Originally posted by @sahuaroWe welcome Sahauro to the Forum.
While you are correct about the context, I think only evangelicals and groups into door-to-door religion use it toward/at non-believers. Otherwise, "gospel preaching" tends to take place in a church, and the audience for that preaching most likely closely matches the audience for the original preaching.
A new poster is often encouraging.
I have only heard this verse being used by evangelists. And sometimes it was in a gospel tract or on a radio program.
It usually is used as an outreach message to those who are opening the door of their heart to Jesus Christ probably for the first time.
There is a good interpretation of Rev. 3:20. And there are suitable applications of Rev. 3:20.
The Holy Spirit speaking to men is so flexible it would be impossible for anyone to dictate how God might use the words to convict someone - Christian or non-believer.
That is the wonderful thing about the Bible. I don't mean that men can use it willy-nilly, but that the Holy Spirit is living and might illuminate the rhema within the logos in many ways.