It is not something that can replace the personal witness to Christ in which we name the name of the saviour. We should assume that the disciples understood that they would tell others about him, but he is asking something more of them. It is something in addition and not in the place of talking about him. It is difficult too for another reason. It has been a popular heresy for many centuries, and it is extremely resistant to the good news.
People often believe that they can save their souls by doing good.
But that is not Christian teaching. As Paul says so strongly in his letters to the young churches, we depend for our salvation, not on what we do but on what God has done for us. It is given to us freely out of God's love for us. It is a gift of grace, not earned, but freely given. But that does not mean that we can do as we like.