- As a follow-up to that, on to an important issue:

In contrast to those who do respond-- there are many other people who would not answer the door even if Jesus knocked, ...and we find that the Scripture explains the reason why:

Before Jesus reaches up to knock on the heart's "doors" of those who don't believe, these people inside are not able to hear the knocking of Christ (nor his voice calling out) because spiritually they are totally deaf.
-- We know this is the case, because Jesus says, "Why don't you understand what I am saying? It is because you are not able to hear my word. ...He who belongs to God hears what God says. The reason you do not hear is that you do not belong to God" (John 8:43, 47).

So, even though Jesus spoke audible words which were clearly heard by the physical ears and brains of unbelievers, Jesus said these people were not able to "hear" (spiritually) so as to choose to respond positively to him, because they didn't belong to God. -- Here Jesus is speaking about inability. --A person must first belong to God before they are able to hear. And unless people's ability is changed by God, the non-believer is not able to hear the "knocking" of Christ, even if he were to knock. - In contrast, however, every one who does belong to God, is able to hear ...and does hear ...and they all respond.

Similarly, before Jesus reaches up to knock on the doors of many people's hearts, these non-believers inside are not able to look through the window of the door and see that Christ is standing there knocking, because spiritually these people are completely blind.
-- We know this is the case, because Jesus says, "I tell you the truth, no one can see the kingdom of God unless he is born again" (John 3:3). Being born again must come before spiritual sight begins, otherwise the non-believer is totally blind to the rightful lordship and ruling authority of Christ and God. - And further, Paul tells us, "The god of this age has blinded the minds of unbelievers, so that they cannot see the light of the gospel" (2Cor. 4:4). - Without first being changed, they can't see, because they are not able to do so.

So, it doesn't matter how brightly or often we as evangelists shine the light of the gospel through any preaching, media or means ...these non-believers are so spiritually blind that they are not able to see the slightest glimmer of the brilliant light of the gospel ...nor the awesome Lord/Savior Jesus standing at their door ...unless people's spiritual ability is changed through first being born again. -- But after being born again, they will be able to see, and they will respond.

Furthermore, even if these people were able to spiritually hear and see, we find in Scripture that the heart's desire is not there to respond to God and Christ --in fact, there is a strongly negative emotion and motivation against God.
-- We know this is true, because Paul tells us that "there is no one who understands; no one who seeks God" (Rom. 3:11). We may suppose that some unbelievers are "seeking God" and desire to know Christ, but here in Romans, Paul informs us that this is never true. In fact, it's even worse: Paul notes that "the sinful mind is hostile to God. It does not submit to God's law, nor can it do so" (Rom. 8:7,8). Again, this is a matter of inability.

Every unsaved person belongs to "the world," and has this "sinful mind" which is hostile toward God. -- Jesus confirms this, saying, "the world cannot hate you, but it hates me" (Jn. 15:23), and even more than this, Jesus says, "He who hates me hates my Father as well" (Jn. 15:23). Paul adds that unsaved people are actually "God's enemies" (Rom. 5:10). Unless they are changed, they are not able to be otherwise.

Now, sometimes Christians assume that through our intelligently logical reasoning, and our love and encouragement, non-believers (without first being born-again and changed) will become able to gain those spiritual abilities they lack, and will thus be able to hear and see, and to start desiring God --and that they will be able to "answer the door" when Jesus knocks.
--But Jeremiah asks: "Can the ...leopard change its spots? Neither can you do good who are accustomed to doing evil" (Jer. 13:23). No. They can't change, and they don't want to change; ...they hate the true God. This is the reason Paul describes the unbeliever's situation this way: "you were dead in your transgressions and sins" (Eph. 2:1).

Thus, before Jesus knocks on a door with an unbeliever behind it, the person inside is not only deaf and blind ...he is spiritually dead. - And in their (unchanged) motivation and free self-will, they want it to stay this way. - - And what kind of response can a dead person give? - - None. - - And how much can a dead person "cooperate" to help someone make them alive again? - - None.

So, what is required for a non-believer to respond positively to Jesus and the good news about him? -- They must be changed before they can "hear" and understand the truth about Christ. They must be given the ability before they can "see" and "believe" the gospel. They must be made alive before they can have faith and trust in Christ, and respond to the gospel.
--- Jesus said, "you do not believe, because you are not my sheep" (Jn. 10:26). - When we read that carefully: The reason a person does not believe/trust in Christ is because they are not one of his people ("sheep"). If a person is not first a "sheep" of Christ's, then they will not believe. --However, if a person is a "sheep," then that is the reason they will believe. Being a sheep comes first before the ability to believe. -- In other words, a person must first be born again, and then they will begin having saving faith.
-- In a nutshell: Regeneration precedes (and initiates) saving faith.

Conveying much the same idea, remember that Jesus said, "The reason you do not hear is that you do not belong to God" (John 8:47). -- A person must first belong to God (be born again) before they can hear and have saving faith. - The reason they are spiritually deaf, is because they do not already belong to God. God must first make them his --raise them spiritually from the dead-- before they can begin to "hear" and respond in faith to God's Word.

- - Again, regeneration precedes (and initiates) saving faith.

So, what causes a person to be born again, so that they can have saving faith and be able to see and hear and come to Christ? What initiates it? -- God must choose to do it by the power of the Holy Spirit. Then a person is "born of the Spirit" (Jn. 3:8). - - - The Gospel of John states, that people who are born as children of God are not born "of human decision ...but born of God" (Jn.1:13). The spiritual birth-event does not happen because unbelieving humans decide they would like it --but because God so decides it first. God's decision and action is what causes someone to be born again. It is not "of human decision." - - Jesus clearly said that, "No one can come to me unless the Father who sent me draws him" (Jn. 6:43). That word "draw" means to "pull" or "drag." This is primarily talking about the ability of humans to come on their own to Christ for salvation. It can't happen. God must first initiate the new birth and cause it to happen. We are not able to come to Christ unless God first pulls us out of death into newness of life, and then we come to him. This is the spiritual "birthing" process, so to speak.

Therefore, at the time when I heard Jesus knocking on my heart's door, I had just previously been born again, and that is the reason I was able to hear. I had just been regenerated, which is the reason I could see and understand the light of the gospel of Christ, and I no longer hated God. Immediately after God gave me new birth, I was able to repent from my sin, and truly desired to come to Christ and make him my Lord ...which is exactly what I freely, willingly and happily did.

God's action to draw me and change me preceeded my decision for Christ. - This is true for all Christians.

- When God looked down through the long corridor of time, he did not see my future faith and therefore choose to save me, --but rather, he saw my future inability to see, believe and respond, and therefore, by grace alone he chose me to be a person he would change and draw to Christ and regenerate, so that as a result I would respond to Christ in true saving faith, choosing him as my saving Lord.

So, what are we evangelists to do? -- Pray. -- Pray that God would change hearts. Pray that he would cause many to be born again so that they can "hear the message" and repent (Acts 5:31; 11:18), and respond (Ac. 16:14) with saving faith (Rom. 10:17).

(An interesting NOTE: Some have suggested about the painting, that the person inside must open the door because there is no door-knob on the outside of the door where Jesus is knocking; but on closer inspection, the door may actually have a knob, but the sleeve of Jesus covers the area where the door-knob would be located, and the right edge of the door has the hinges, where no door-knob would be.)

All glory to God alone.