Here Jesus knocks not at the door of the individual sinner, but rather at the door of a church that was acting like one! Whereas Jesus had set before another church an open door, inviting them into his presence despite the false accusations of their persecutors (Rev 3:8), he was here locked out of another church. Ancient hospitality required sharing food with a guest, but the Laodicean church had locked Jesus out by their arrogant self-sufficiency (3:17-18). He wanted these Christians to repent and express again their need for him (3:19).
This does not make illegitimate the faith of those led to Christ using this verse; the principle applies, and it is in any case the gospel message, not the interpretation of a verse, that converted them. But the point remains that if we misinterpret the verse, we do not learn what this passage has to say to us. There may be arrogant churches today that have locked Jesus out.
