This is not justified as "made just" (like us sinners), but justified from all slanders. Vindicated or defended by the Holy Spirit, would be the idea here. Let see some examples.
The miracles that Jesus did were by the power of the Spirit
The Spirit of the Lord is upon me, because he hath anointed me to preach the gospel to the poor; he hath sent me to heal the brokenhearted, to preach deliverance to the captives, and recovering of sight to the blind, to set at liberty them that are bruised, to preach the acceptable year of the Lord. [Luke 4:18-19]
But if I cast out devils by the Spirit of God, then the kingdom of God is come unto you. [Matthew 12:28]
After suffering on the cross, Christ was raised from the dead by the Holy Spirit.
But if the Spirit of him that raised up Jesus from the dead dwell in you, he that raised up Christ from the dead shall also quicken your mortal bodies by his Spirit that dwelleth in you. [Romans 8:11]
And declared to be the Son of God with power, according to the Spirit of holiness, by the resurrection from the dead [Romans 1:4]
The teaching of the Apostles comes from the Spirit
But the Comforter, which is the Holy Ghost, whom the Father will send in my name, he shall teach you all things, and bring all things to your remembrance, whatsoever I have said unto you. [John 14:26]
And all the miracles that the Apostles performed in the name of Christ were by the same Spirit.
For I will not dare to speak of any of those things which Christ hath not wrought by me, to make the Gentiles obedient, by word and deed, through mighty signs and wonders, by the power of the Spirit of God; so that from Jerusalem, and round about unto Illyricum, I have fully preached the gospel of Christ. [Romans 15:18-19]
In all this Christ is placed in His exalted place, thanks to the work of the Holy Spirit. Not only then, but also now. All these actions combined:
- God was manifest in the flesh;
- Justified in the Spirit;
- Seen of angels;
- Preached unto the Gentiles;
- Believed on in the world;
- Received up into glory.
make our godliness possible. And that's the mystery, how can we become godly? Through the work of Christ, here summarized by Paul.