This is the good news of the gospel; that God, through Jesus Christ's sacrifice, now knocks on the doors of our hearts. Whosoever will may answer. Take note of who's doing the knocking in this verse -- Him. As long as we're responding to His knock, it's a good sign. If we're knocking at the door ... uh ... not so good sign.

When we do the knocking, you get what happens in Matthew 25:11-12, when the five foolish virgins found the door closed because they failed to prepare themselves while it was open. " 'Sir! Sir!' they said. 'Open the door for us!' ¹²"But he replied, 'I tell you the truth, I don't know you.' " And again, in Luke 13:24-27, we read, ²⁴"Make every effort to enter through the narrow door, because many, I tell you, will try to enter and will not be able to. ²⁵Once the owner of the house gets up and closes the door, you will stand outside knocking and pleading, 'Sir, open the door for us.'  "But he will answer, 'I don't know you or where you come from.' ²⁶"Then you will say, 'We ate and drank with you, and you taught in our streets.' ²⁷"But he will reply, 'I don't know you or where you come from. Away from me, all you evildoers!'

I dare say, these were they who heard His knock and didn't respond to it until it was too late. So let us respond to His knock today, in the day of salvation (2 Corinthians 6:2), and not harden our hearts (Hebrews 3:7-8 and 15)