The apostles were specifically picked by Jesus to deliver His message and write it down (either through Luke or Mark) for generation to next generation. The Apostles were His 12 disciples that walked with Jesus on earth and were specifically taught by Jesus prior to His death on the Cross and before he ascended in to heaven. Paul was the last apostle to see Jesus...and this was very special because Jesus took Paul to Heaven to discuss his mission.
The Holy Spirit works in ALL believers to guide them in the truth, bringing remembrance of God's Holy Word...which is Christ, praying for them when human words are not enough. The Holy Spirit seals the believer, never leaving the believer even when the believer intentionally sins, but will bring the believer to his knees in repentance for the sins he committed. The Holy Spirit provides the power to walk upright.