Let’s read a couple of verses that pertain to your good question.  First, we read the words of the Lord in Luke 24:49, “And, behold, I send the promise of my Father upon you: but tarry ye in the city of Jerusalem, until ye be ENDUED WITH POWER FROM ON HIGH.”  Now let us read Acts 1:1-5, “The former treatise have I made, O Theophilus, of all that Jesus began both to do and teach, Until the day in which he was taken up, after that he through the Holy Ghost had given commandments unto the apostles whom he had chosen: To whom also he shewed himself alive after his passion by many infallible proofs, being seen of them forty days, and speaking of the things pertaining to the kingdom of God: And, being assembled together with them, commanded them that they should not depart from Jerusalem, but wait for the promise of the Father, which, saith he, ye have heard of me. For John truly baptized with water; but ye shall be BAPTIZED WITH THE HOLY GHOST not many days hence.”

Along with these verses, let’s read Acts 2:1-4, “And when the day of Pentecost was fully come, they were all with one accord in one place. And suddenly there came a sound from heaven as of a rushing mighty wind, and it filled all the house where they were sitting. And there appeared unto them cloven tongues like as of fire, and it sat upon each of them. And they were all filled with the Holy Ghost, and began to speak with other tongues, as the Spirit gave them utterance.” 

It was on the day of Pentecost that the Holy Spirit descended to the earth and indwelt EVERY BELIEVER and sealed each one until that day of our departure from the earth.  As we just read, “they were ALL filled with the Holy Ghost.”  You are correct in saying that the Spirit was already at work in the world throughout the Old Testament.  Yet, in the Old Testament, the Holy Spirit did not indwell every believer, neither did He seal each believer with Himself. 

Let’s consider King David’s prayer of repentance and confession in Psalm 51:10-12 where he prayed, “Create in me a clean heart, O God; and renew a right spirit within me. Cast me not away from thy presence; and take not thy holy spirit from me. Restore unto me the joy of thy salvation; and uphold me with thy free spirit.”  In this prayer, David prayed six points.  Today, believers might well pray the same thing in our prayers of confession EXCEPT the middle two points.  “Cast me not away from thy presence; and take not thy holy spirit from me.”  We do not ask the Lord to not cast us away because He has promised He would never do that to us.  We read in Hebrews 13:5, “…for he hath said, I WILL NEVER LEAVE THEE, NOR FORSAKE THEE.”  We do not ask the Lord to not take the Holy Spirit from us because He has SEALED us with His Spirit as we read in Ephesians 1:13-14, “In whom ye also trusted, after that ye heard the word of truth, the gospel of your salvation: in whom also after that ye believed, YE WERE SEALED WITH THAT HOLY SPIRIT OF PROMISE, Which is the earnest of our inheritance until the redemption of the purchased possession, unto the praise of his glory.” 

The Lord Jesus spoke of the coming of the Holy Spirit to indwell each believer in John 14:26, “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.”  He also said in John 16:7, “Nevertheless I tell you the truth; It is expedient for you that I go away: for if I go not away, the Comforter will not come unto you; but if I depart, I will send him unto you.”  It was by the Lord’s departure…His ascension to Heaven after His death, burial, and resurrection that they would be led by the Holy Spirit to see more fully the design of his coming than they would by his presence.  It was the abiding presence of the Spirit in each believer that allows Him to lead us into all truth and understanding.  (CC)  (597.2)