From Tozer:

There is another kind of evidence. It is the immediate evidence of the inner life. That is the evidence by which you know you are alive. If I were to prove that you weren’t alive, you would chuckle and go home just as alive as you are now and not a bit worried about it, because you have the instant, unmediated evidence of internal life. Jesus Christ wanted to take religion out of the external and make it internal and put it on the same level as life itself, so that a man knows he knows God the same as he knows he is himself and not somebody else. He knows he knows God the same as he knows he is alive and not dead. Only the Holy Ghost can do that. The Holy Spirit came to carry the evidence of Christianity from the books of apologetics into the human heart, and that is exactly what He does.

Tozer, A. W. How to Be Filled with the Holy Spirit (Kindle Locations 232-238). Kindle Edition.