"Behold, I stand at the door and knock..." (cf. Rev. 3:20).
Is Jesus really knocking at the door of the heart of the unbeliever, as evangelists ostensibly quote Rev. 3:20 in their salvation formula? This verse is addressed to the church of Laodicea, a so-called "lukewarm" fellowship, that had apparently no room in their hearts for Jesus. What is delineated by lukewarm is that Jesus didn't know where they stood spiritually; they wouldn't take a stand for Him and proclaim His name, but were sitting on the fence trying to remain neutral. Jesus would rather mingle with people who are not believers (even publicans, sinners, and any so-called scum) than pretenders or hypocrites who feign belief.
God doesn't require perfect and complete faith, but sincere, unfeigned faith. True faith is not ashamed to proclaim Jesus as Lord openly and to spread the good news. True faith is contagious and people catch on and wonder what makes you different. Jesus said that if you are not for Him, you are against Him, and that means lukewarm professors (and there is a difference between a profession of faith and reality of faith) are not believers and are in the same boat as unbelievers.
Jesus didn't say that if you're not for Him you are neutral or against Him! But against Him without a doubt. The problem with this church is that they had an outward show of religion, a memorizing of the Dance of the Pious, and no inward reality. The true believer desires to live out His faith and to know Christ, as well as make Him known--possessing a burden for the Great Commission and the Great Commandment.
This verse applies to salvation, but it is not the whole equation. The big picture is that you must believe in your heart and not have a story or head belief to be a genuine believer. Faith is not mere mental assent or acquiescence! Faith is something put into action, viz., knowledge and knowledge acted upon and demonstrated. This church had not gone the route of loving Jesus with their whole heart and were only halfhearted followers, not having a vital fellowship and relationship with the Lord, which is what Christianity is all about--not a philosophy, but getting to know Jesus and how He thinks and fellowshipping with Him, the Father, and in the church. Christ cannot open the door, the resident must and He desires true fellowship with us--the invitation is to all who hear His voice, and the sheep will hear His voice (cf. John 10:27).
In summation, saving faith is not believing merely in the Jesus of history, but in the one knocking at the door! Someone has said: Faith is not believing that there is a God, but in the God who is there! Jesus must not be just born, but born in you! Soli Deo Gloria!