Friend, when you are sitting at the feet of Jesus you have the Holy Spirit speaking to you. The Father and the Son both have the same Spirit. And we are given assurance of this when Jesus was baptized. The Holy Spirit descended upon the Lord in a visible manner, that everyone could see that he had the Holy Spirit. John (the baptist) even testifies of this event. Yes, absolutely! The Holy Spirit is necessary for any man to understand the truth of God and we also know that before the first apostles were sent out into the world to fulfill their commission, that the Holy Spirit also came down upon them in a visible display that people then and people now would know that they had the Holy Spirit.
Now, one might argue that, "Well, then they didn't really understand all the truth of God until Pentecost." And I would give some creedence to that understanding because of the many times that Jesus rebuked them for being dull and unable to understand, but even that allows that by the time they went out and began using the 'keys' of the kingdom that Jesus had given unto them in his teaching, that they did fully understand what it was they were teaching and believing as the truth. Because they then had the indwelling Holy Spirit! This is part of my argument that we need to be very careful in accepting the words of men after these that try to tell us what it is that God desires of us. I don't have any such assurance that someone 100 years or 200 years later has the indwelling Holy Spirit to understand the things of God, as I do of the first apostles.
As a matter of fact, it was to these very men that Jesus said, "When the Holy Spirit comes to you, he will lead you into all truth." He then instructed them to wait for the Holy Spirit before they began their task of teaching and preaching the 'keys' to the kingdom. So, yes, I would agree that there may well have been some parts of Jesus' teaching to them that they may not have fully understood until the day of Pentecost, but not before they began to use the 'keys' to the kingdom that Jesus had given them that, yes, I understand was the wisdom and knowledge of the things of God that he shared with them.
I hope that you fully appreciate the time that Jesus spent with his apostles. This was not some 9 to 5 job where everybody got in their cars and went home to their families at the end of the day. This band of men traveled together, day and night, all over Israel learning at the feet of the Savior. Surely, in the evening, as they ate and readied themselves for a night's rest they spoke to one another and asked many questions and received many answers. When they traveled from town to town as a group, Jesus spoke to them of many truths and explained to them many things that other people of that day were not privey to. We have at least two times that we are told Jesus sat with his disciples privately and explained to them a parable that he had told the people.
Now, in three years, I imagine that this happened a lot!! So, yes, these men knew the truth and it had all been explained to them, but the total and complete comprehension of all that Jesus had told them may not have really sunk in and been made a part of 'who they were' until the day of Pentecost. Obviously, at the time of Jesus' trial, Peter didn't fully understand who Jesus was because he denied knowing him, but by the time Peter was martyred for his faith, I doubt that he even batted an eye or took one moment of hesitation in explaining to his persecutors that, yes, he knew Jesus and he stood with him.
God bless you.
IN Christ, Ted