A Christian worker once visited a poor woman in order to bring her a sum of money which she badly needed. He knocked at her door, not once or twice only, but got no response & he had to go away disappointed with the money still in his pocket.
Meeting her a few days later, he told her of his fruitless visit. She replied, with tears in her eyes, that she had heard him knocking, but thought he was the rent collector & she dare not open the door, for she had not the money to pay the rent.
C. H. Spurgeon used this as an illustration of the Gospel. He who stands at the door & knocks has come with a gift; the knock is the Savior's knock, not that of a taskmaster. The Gospel is good news; news of a debt paid, not of rent required. It is not a demand but a free gift, which no one has deserved or could win by merit, but which is offered to all.
Rev 3:20 (CEV): "Listen! I am standing and knocking at your door. If you hear my voice and open the door, I will come in and we will eat together."
Rom 3:24 (KJV): "Being justified freely by his grace through the redemption that is in Christ Jesus."