The Holy Spirit is often the part of the Trinity that we sometimes give less attention to or don't talk about enough.
The work of the Holy Spirit is extremely important, and without it we would ultimately have no gospel and no New Testament in the world today.
When Jesus left the world and spoke parting words to His disciples on Olivet, He told His disciples to continue spreading the Word and bear witness to all the nations about Him. These were His final words and command to the disciples.
Acts 1:8 (KJV): 8 But ye shall receive power, after that the Holy Ghost is come upon you: and ye shall be witnesses unto me both in Jerusalem, and in all Judaea, and in Samaria, and unto the uttermost part of the earth
How good could they ultimately be at completing this task?
For one, you could say they were average pupils, who did learn, but consistently failed to understand His teachings and missed the points throughout His ministry until He explained things to them in further detail. This was ultimately a learning experience for all from Christ. Was it reasonable to expect them to be able to continue His mission and spread the Truth as He spoke it flawlessly and not get anything mixed or muddled up over time?
The answer is no - so Jesus sent the Holy Spirit to teach them the Truth and guide them in all of their ways, so that they would be error free, and never forget what they had already been taught and to show them new things that the Lord intended to be revealed to them in due time.
John 14:26 (KJV): 26 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 16:12-15 (NKJV): 12 “I still have many things to say to you, but you cannot bear them now. 13 However, when He, the Spirit of truth, has come, He will guide you into all truth; for He will not speak on His own authority, but whatever He hears He will speak; and He will tell you things to come. 14 He will glorify Me, for He will take of what is Mine and declare it to you. 15 All things that the Father has are Mine. Therefore I said that He [a]will take of Mine and declare it to you.
Then it happened, just as He foretold it would - the Holy Spirit revealed itself to the disciples:
1 Corinthians 2:9-13: 9 But as it is written: “Eye has not seen, nor ear heard, Nor have entered into the heart of man The things which God has prepared for those who love Him.” 10 But God has revealed them to us through His Spirit. For the Spirit searches all things, yes, the deep things of God. 11 For what man knows the things of a man except the spirit of the man which is in him? Even so no one knows the things of God except the Spirit of God. 12 Now we have received, not the spirit of the world, but the Spirit who is from God, that we might know the things that have been freely given to us by God. 13 These things we also speak, not in words which man’s wisdom teaches but which the Holy Spirit teaches, comparing spiritual things with spiritual.
One could go even further and say that Christians in general would be scarce to non-existent if not for the continued presence of the Holy Spirit. We need the Holy Spirit to show the world Christianity - just as Christ told Nicodemus in John 3:11, "You people do not accept our testimony", the gospel produced no conviction in those people because unbelief holds them fast, so is this the same battle we face today trying to spread the word of our King Jesus to others.
We need the Holy Spirit to renew a blinded heart. It is the sovereign prerogative of Jesus' Spirit to convince man of the truth of Jesus' gospel, and us believers must learn to ground our hopes of success not in clever presentation, but on powerful demonstration of the truth by the Spirit.
1 Corinthians 2 (NKJV): 2 And I, brethren, when I came to you, did not come with excellence of speech or of wisdom declaring to you the [a]testimony of God. 2 For I determined not to know anything among you except Jesus Christ and Him crucified. 3 I was with you in weakness, in fear, and in much trembling. 4 And my speech and my preaching were not with persuasive words of human wisdom, but in demonstration of the Spirit and of power, 5 that your faith should not be in the wisdom of men but in the power of God.
We should be asking ourselves if we honor the Holy Spirit.
Do we acknowledge the authority of the Bible?
Do we apply it to our life and live by it?
Do we trust the Holy Spirit to authenticate our witness unto others?