God has to destroy our determined confidence in our own convictions. "I know this is what I should do" – and suddenly the voice of God speaks in a way that overwhelms us by revealing the depths of our ignorance. We have shown our ignorance of Him in the very way we determined to serve Him. We serve Jesus in a spirit that is not His, we hurt Him by our advocacy for Him, we push His claims in the spirit of the devil. Our words sound all right, but our spirit is that of an enemy. "He rebuked them, and said, Ye know not what manner of spirit ye are of." The spirit of our Lord in an advocate of His is described in 1 Corinthians 13.
Paul wrote it best:
"For we are the circumcision, which worship God in the spirit, and rejoice in Christ Jesus, and have no confidence in the flesh." (Philippians 3:3)
Why should we? He provides us all things through the death and resurrection of His Son, Jesus Christ.
"We serve Jesus in a spirit that is not His." Such a statement is not possible. If we are not serving the Lord in the Spirit, then we are serving Him in our own efforts, in the flesh. We have fallen from grace (cf Galatians 5:4), and no one can be justified by the flesh (cf Galatians 2:!6)