Satan hardens hearts.
The Devil fights evangelism by hardening the hearts of unbelievers. Such hearts are enclosed in hard rock to repel any evangelistic efforts. These hard rock hearts are impenetrable to simple preaching. As you preach, it becomes difficult for your words to enter such hearts. You end up not changing anyone.
The weapon against hard souls.
The only weapon that can penetrate such hard hearts is the written Word of God. The word of God is very powerful. It is very sharp. It is sharper than a double edged sword you have ever known of. As it comes out of your mouth, the written Word of God is accompanied by fire that enables it to penetrate even the hardest hearts and most difficult persons.
Powerful verses that pierce hard hearts.
“Jesus replied, “Very truly I tell you, no one can see the Kingdom of God unless they are born again” John 3:3
“Very truly I tell you, whoever hears my word and believes him who sent me has eternal life and will not be judged but has crossed over from death to life.” John 5:24
“I am the bread of life. Whoever comes to me will never go hungry, and whoever believes in me will never be thirsty” John 6:35
“I am the light of the world. Whoever follows me will never walk in darkness, but will have the light of life” John 8:12
“I am the resurrection and the life. The one who believes in me will live, even though they will die”. John 11:25
“I am the way and the truth and the life. No one comes to the Father except through me” John 14:6
As you preach, keep on quoting the written Word of God from the Bible. It will eventually change the hard hearts.