"Now we have received ... the spirit which is of God; that we might know the things that are freely given to us of God" (1 Corinthians 2:12)
The witness of the Spirit is a sacred inner thing which cannot be explained. It is altogether personal and cannot be passed from one to another. The outward ear cannot hear what it says. Much less can the worldly onlooker know what is taking place.
The Holy Spirit whispers its mysterious Presence to the heart, and the heart knows without knowing how it knows. Just as we know we are alive by unmediated knowledge and without recourse to proof, so we know we are alive in the Holy Spirit. The witness is in the hidden regions of the spirit, too deep for proof, where external evidence is invalid and "signs" are of no use.
When all is said, it may easily be that the great difference between professing Christians (the important difference in this day) is between those who have reduced Christianity to an intellectual formula and those who believe that the true essence of our faith lies in the supernatural workings of the Spirit in a region of the soul not accessible to mere reason.
Show me a man who makes the law of God the rule, and the glory of God the end of his conduct, and I will show you one whose heart has been sanctified by the Spirit of grace.
~A. W. Tozer~