Not by might, nor by power, but by My spirit, saith the LORD of hosts.- Zechariah 4:6

(M)en’s hearts are very hard to affect. If you want to get at them for any worldly object, you can do it. A cheating world can win man’s heart; a little gold can win man’s heart; a trump of fame and a little clamor of applause can win man’s heart. But there is not a minister breathing that can win man’s heart himself. He can win his ears and make them listen; he can win his eyes, and fix those eyes upon him; he can win the attention, but the heart is very slippery. Yes! The heart is a fish that troubles all gospel fishermen to hold. You may sometimes pull it almost all out of the water; but, slimy as an eel, it slippeth between your fingers, and you have not captured it after all. Many a man has fancied that he has caught the heart, but has been disappointed. It would take a strong hunter to overtake the hart on the mountains. It is too fleet for human foot to approach. The Spirit alone has power over man’s heart. Do you ever try your power on a heart? If any man thinks that a minister can convert the soul, I wish he would try. Let him go and be a Sabbath School teacher. He shall take his class, he shall have the best books that can be obtained, he shall have the best rules, he shall draw his lines of circumvallation about his spiritual Sebastopol, he shall take the best boy in his class, and if he is not tired in a week I shall be very much mistaken. Let him spend four or five Sabbaths in trying; but he will say, “the young fellow is incorrigible.”  He will soon find “it is not by might nor power, but by My Spirit, saith the Lord.” He can give a sense of blood-bought pardon that shall dissolve a heart of stone. He can

“Speak with that voice which wakes the dead
And bids the sinner rise;
And makes the guilty conscience dread
The death that never dies.”

He can make Sinai’s thunders audible; yea, and He can make the sweet whisperings of Calvary enter into the soul. He has power over the heart of man. And here is a glorious proof of the omnipotence of the Spirit that He has rule over the heart.~ C.H. Spurgeon

https://www.blueletterbible.org/Comm/spurgeon_charles/sermons/0030.cfm