I will pose this simple question: if we only do good things, will people know about Jesus?
Did you answer ‘yes’? Then I will ask you: do all good deeds, no matter who does them, testify to Jesus? Do the actions of a secular humanist or Buddhist tell people that Jesus died for their sins?
If you said ‘yes’ again, then why did Jesus preach? Consider this:
How then will they call on him in whom they have not believed? And how are they to believe in him of whom they have never heard? And how are they to hear without someone preaching? And how are they to preach unless they are sent? As it is written, “How beautiful are the feet of those who preach the good news!” But they have not all obeyed the gospel. For Isaiah says, “Lord, who has believed what he has heard from us?” So faith comes from hearing, and hearing through the word of Christ. (Romans 10:14-17 emphasis added)