Did the death of Jesus have any effect on all the sins committed under the Old Covenant? Of course it did, in fact Jesus' death cleansed the sins of all mankind who had faith all the way back to Adam and Eve - AND all the way up to His 2nd Coming. So you could say the benefit of the cross encompassed all of time - past, present, and future. Jesus Christ was slain from the foundation of the world. Rev.13:8
Another benefit of the cross was the New Covenant. Is it possible that the benefits of the New Covenant could be experienced before His death, just as His blood cleansed the sins of all who had faith BEFORE HE WAS CRUCIFIED?
What were the benefits of the New Covenant? We see them in Jeremiah 31:31-34.
1. "I will put My law within them (the Law of Christ - Gal.6:2) and on their heart I will write it; Jer.31:33
Did this happen to the disciples during Jesus' life time or did they have to wait until He died? Yes, it happened - No, they didn't have to wait.
Jesus spoke of His disciples when He said, "They have kept Your word". John 17:6; "for the words which You gave Me I have given to them; and they received them and truly understood that I came forth from You, and they believed that You sent Me." John 17:8; "I have given them Your word" John 17:14 "They are not of this world, even as I am not of this world" John 17:14
2. "and I will be their God, and they shall be My people." Jer.31:33
Did this happen to the disciples during Jesus' lifetime? Yes!
"Thomas ... said to Him, 'My Lord and My God!' " John 20:28
Peter said, "by the righteousness of our God and Savior, Jesus Christ;" 2 Peter 1:1
Matthew said, "and they shall call His name Emmanuel, which ... means 'God with us.' " Matthew 1:23
John said that Jesus "was calling God His own Father, making Himself equal with God." John 5:18
"Who is My mother and who are My brothers? And stretching out His hand toward His disciples, He said, 'Behold My mother and My brothers. For whoever does the will of My Father who is in heaven, he is My brother and sister and mother." Matthew 12:48-50
"But he who enters by the door is a shepherd of the sheep. To him the doorkeeper opens, and the sheep hear His voice, and he calls his own sheep by name and leads them out. When he puts forth all his own, he goes ahead of them, and the sheep follow him because they know his voice." John 10:2-4
3. "For they will all know Me" Jer.31:34
Did this happen to the disciples during Jesus' lifetime? Yes.
"I am the good shepherd, and I know My own and My own know Me". John 10:14
"even as You gave Him authority over all flesh, that to all whom You have given Him, He may give eternal life. This is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent." John 17:2-3
"My sheep hear My voice, and I know them, and they follow Me;" John 10:27
4. "for I will forgive their iniquity, and their sin I will remember no more." Jer.31:34
Did this happen to the disciples during Jesus' lifetime? Yes.
Jesus said to the paralytic, "Take courage son; your sins are forgiven." Matthew 9:2
Jesus said to the woman who wet His feet with her tears and wiped His feet with her hair - "Your sins have been forgiven." Luke 7:47
"But so that you may know that the Son of Man has authority on earth (obviously, during His lifetime) to forgive sins ..." Matthew 9:6
"Our Father ... forgive us our debts, as we also have been forgiven our debtors." Matthew 6:12
"Today salvation has come to this house, because he too is a son of Abraham. For the Son of Man has come to seek and to save that which was lost." Obviously, Zaccheus' sins were forgiven. Luke 19:9-10
So ALL of the earmarks of the New Covenant were there during Jesus' lifetime. His death purchased or validated it, but it had already begun. The New Covenant would have no strength or force if Jesus had not died. But His death was a foregone conclusion, so it started when the Jesus said it did:
"For all the prophets and the Law prophesied until John." Matthew 11:13
"The Law and the Prophets (were proclaimed) until John; since that time the gospel of the kingdom of God has been preached ..." Luke 16:16
Hebrews 9:16-17 The death of Christ was retroactive, that is forgiveness of sins, all the way back to Adam and Eve. So it is true with the New Covenant. It began when Jesus said it did: Mat.11:13 and Luke 16:16. This Hebrews passage is not and cannot be saying that the New Covenant cannot begin until Jesus' death. Rather it seems to be saying that if Jesus had not died for our sins, there would not have even been a New Covenant.
When Jesus said, "Repent for the the kingdom of God is at hand."(Mat.4:17), He was NOT saying it was 3 1/2 years later, He was saying, "It is NOW", just as close as your hand.
Again, this is why the 4 gospels are the first books of and in the New Covenant. They are New Covenant books, not Old Covenant books. Everything in them takes place during the New Covenant.
What about the Holy Spirit? Well, Jesus said that He cast out demons by the Spirit of God and therefore "the Kingdom of God has come upon you". Matthew 12:28 Jesus confirms again that the Kingdom of God WAS ALREADY THERE.
But how did the apostles cast out demons, when Jesus sent them out? Matthew 10:1 Yes, by the authority that Jesus gave them, but also by the Spirit of God.
"Master, we saw someone casting out demons in Your name ... Jesus said ... Do not hinder him ... Luke 9:49
How did this man cast out demons? In Jesus' name, by the Spirit of God.
So the Spirit of God was present and active during Jesus' lifetime, and NOT just in HIM alone - He was active in His 12 disciples and also in others who by faith, performed miracles. Mark 9:39
So, given this information, we know that the Baptism of the Holy Spirit, which occurred 10 days after His ascension, was something other than what the disciples had already experienced in their performing of miracles during Jesus' lifetime. Or another possibility could be that the Baptism of the Holy Spirit was available to ALL believers, whereas before Pentecost, the power of the Holy Spirit was given to just a few.