This is not the 1st Lord's day of the catechism that deals with the Holy Spirit. We were taught of the Trinity of God and that the Holy Spirit is very God of very God. LD20 finds us at the end of the Apostolic confession and the subject of the WORK of the Holy Spirit among the church. The Spirit is the Spirit of our Lord Jesus Christ. Secondly, the Holy Spirit so works so that the benefits of Christ are applied by the Holy Spirit in His sanctifying both directing and guiding us in all the way of the truth!
Who He is: He is co-equal with God the Father and God the Son. He is very God of very God. Some believe the Holy Spirit is an emanating POWER and thus deny the Trinity! Read the story of Ananias and Sapphira where they conspired and lied to the 3rd person of the triune God. The Holy Spirit in the old testament is distinguished from the Spirit in the new testament just like Jesus the Son and the Son incarnate. The Holy Spirit was not yet in the old testament. He did exist and work, but did so in the background through types and shadows as a schoolmaster that brought them to Christ. Kept at arms length as the people sacrificed in the temple opposed to being the people being LIVING sacrifices. The accuser always asked where is the atonement? And all the people in heaven rejoiced as Christ died, paid for, and atoned for their sins. It had finally been sealed and now He works WITHIN us after the Spirit was poured out.
What He does: He has been given to Christ to work in the lives of His children so that application of His blessings are concrete. The Holy Spirit is ALWAYS self effacing and as Christ said "He ALWAYS bears witness of ME!" Those churches where the Spirit is on the fore are NOT correct. He ALWAYS speaks of Christ and never does the Spirit "toot His own horn." The gift of the Holy Spirit is not the waving of hands or the speaking of tongues, but the TESTIFYING to me of CHRIST! The Spirit works in the hearts and lives of the child of God and you can see the RESULT of His work but do not see the Holy Spirit. He works unto a SPIRITUAL resurrection starting in regeneration the very first work of the Holy Spirit when He touches us spiritually and gives us an aptitude for SPIRTUAL things. Do you view your children that YOU can persuade them in order for them to be saved? Or do we believe that God knows His children from all of eternity and brings them into His family through the teaching of God fearing parents whereby He touches them and binds the truth unto their hearts!?! That is how Zion is built in the midst of this world. And we may not make the error of separating the Holy Spirit and the WORD as He has bound Himself to the WORD in ALL His work with the exception of regeneration. He continues to lead and guide us in all the way of the truth!
Our abiding comfort: Christ tells the disciples about the Comforter and that it was expedient that He leave them for a little while. Our Lord Jesus Christ dwells IN YOU and me and all His people throughout history. He abides in us, in our succor, and will not leave us. But He leads us in the Word of God as He leads and guides us till we are in heaven where we will no longer need to be comforted.