A.W. Tozer emphasizes that the fullness of the Spirit in a believer's life is often misjudged by emotional experiences or physical manifestations, such as speaking in tongues. He argues that true evidence of the Spirit's work is not found in outward signs but in the moral and behavioral changes that occur in a person's life after conversion. These changes serve as collateral proof of one's faith but do not constitute the internal witness of the Spirit. Tozer highlights the importance of understanding the relationship between the human spirit and the Holy Spirit, asserting that genuine transformation is the true mark of being filled with the Spirit.

Again, the experience of the Spirit's fullness coming upon the believer's heart is often judged by the amount and quality of emotional charge that accompanies it. Some go so far as to declare bluntly that no one is filled with the Spirit who has not experienced certain physical phenomena, particularly the act of speaking in unidentified tongues. Others will settle for an increased degree of joy or more effectiveness in their service. All this is wrong, both scripturally and psychologically. It is the result of a misunderstanding of the nature of man's soul and of the relation of the spirit of man to the Spirit of God. The workings of God in the hearts of redeemed men always over flow into observable conduct. Certain moral changes will take place immediately in the life of the new convert. A moral revolution without will accompany the spiritual revolution that has occurred within. As the evangelists tell us, even the cat will know it when the head of the house is converted. And the grocer will know it too, and the old cronies in the haunts where the man used to hang out will suspect that something has happened when they miss the new Christian from his accustomed place. All this is collateral proof of the validity of the man's Christian profession. But it is in no sense evidence to the man's own heart. It is not the witness of the Spirit.