I think that is the case. The Covenant of Law presented in shadow, in parable, the Atonement that would be produced through the sinless Son of God. While the Mystery of Christ was not made known until after Pentecost and the ministry of the Comforter had begun, obedience to God in relation to this Covenant was not mandated for no purpose. They understood sacrifice for sin, and while it was not until the Comforter made this understandable to man, their faith resulted in obedience through faith in God and a willingness to obey.
Consider:
1 Corinthians 2:7-11
King James Version (KJV)
7 But we speak the wisdom of God in a mystery, even the hidden wisdom, which God ordained before the world unto our glory:
8 Which none of the princes of this world knew: for had they known it, they would not have crucified the Lord of glory.
9 But as it is written, Eye hath not seen, nor ear heard, neither have entered into the heart of man, the things which God hath prepared for them that love him.
10 But God hath revealed them unto us by his Spirit: for the Spirit searcheth all things, yea, the deep things of God.
11 For what man knoweth the things of a man, save the spirit of man which is in him? even so the things of God knoweth no man, but the Spirit of God.
In v.9 we see this quotation applied to Christ. If those that ultimately contributed to the death of Christ had known this mystery, that is, what Christ would accomplish through His death and Resurrection, they would not have crucified Him, but, I believe, they and Satan would have assigned a security detail to make sure He would not even dash His foot against a stone.
Romans 16:25
King James Version (KJV)
25 Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began,
If we carefully consider the Disciples in the Gospels, it becomes clear that they do not understand the Mystery of Christ, though it was not given unto them to understand at that point. Christ told them directly that He would die, Peter going so far as to say "This will never happen to you," which statement earned him a severe rebuke from the Lord.
Paul tells us this mystery was kept secret since the world began.
Concerning those given unto the Lord, we see that He kept them and none were lost:
John 17:12
King James Version (KJV)
12 While I was with them in the world, I kept them in thy name: those that thou gavest me I have kept, and none of them is lost, but the son of perdition; that the scripture might be fulfilled.
They were kept...by the Lord. However, two important things are clear in scripture: 1) the Lord said He must leave that the Comforter come; 2) the disciples had not received the Spirit of God, and this is clear in Acts 1, where the Lord tells them the Promise of the Father would come, and they would be baptized with the Holy Ghost:
Acts 1
King James Version (KJV)
4 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.
5 For John truly baptized with water; but ye shall be baptized with the Holy Ghost not many days hence.
6 When they therefore were come together, they asked of him, saying, Lord, wilt thou at this time restore again the kingdom to Israel?
7 And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power.
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.
Even at this point the disciples did not understand fully that which the Lord had done. But, not many days hence...they would.
I do as well. Just as the disciples were kept by Christ before the Cross, even so those that shall be heirs of salvation are as well. The disciples had not received the Comforter at that point but we can see that the ministry of Christ did have impact on their lives:
John 15
King James Version (KJV)
3 Now ye are clean through the word which I have spoken unto you.
Ephesians 5:25-26
King James Version (KJV)
25 Husbands, love your wives, even as Christ also loved the church, and gave himself for it;
26 That he might sanctify and cleanse it with the washing of water by the word,
Romans 10:17
King James Version (KJV)
17 So then faith cometh by hearing, and hearing by the word of God.
Faith in Christ arises through the word, and specifically the word introduced to the heart of man by the Holy Spirit, as Paul teaches that the natural man cannot understand the things of God.
I would suggest that prior to the Cross and Pentecost the specific nature of the ministry of Christ was not revealed, even to His closest disciples. The cleansing which the word of God has always produced did not begin after Pentecost, however, it was not until God made known to man through the ministry of the Comforter the mystery of Christ that it became understood. We can look into the Law and clearly see Christ, far more clearly than the Old Testament Saints. But they were still assured of salvation though their faith did not have the revealed focus of the Son of God dying on behalf of man, to take upon Himself our penalty for sin.
I believe it was exactly that.
Galatians 3:6
King James Version (KJV)
6 Even as Abraham believed God, and it was accounted to him for righteousness.
But Abraham could not place his faith in the Risen Lord.
As posted previously, many Old Testament Saints received "a good report." However, they were not made perfect, and the importance of that cannot be underestimated.
The Law and the sacrifices of Law could not make perfect, or complete, only Christ fulfilling that which the Law only pictured could. Abraham had faith in God, but you and I can place our faith in the completed work of Christ, Who has died in our place that we might have life. That life, Eternal Life, is not just that we will live forever, but because God is in us, His Life is in us.
The saints of old can be seen to have faith, and in fact can be seen to have been moved by the Spirit of God, Who worked through them, however, concerning the ministry of the Holy Spirit in our lives, we are told, as Percho quotes in the previous post, that He is the earnest of our inheritance. We see King Saul as an example of one through whom the Spirit of God worked through yet the Spirit of God departed from him. There is no indication of this scripture concerning those born again, and in fact, we are told the exact opposite, and are sure He will never leave nor forsake us.
It is my belief as well that God makes provision for those that cannot fully comprehend sin, righteousness, and judgment. This would include children and those mentally challenged. God is righteous and will not demand something that He has not also made clear to an individual. This can be seen in those before the Cross, who did not understand the Mystery of Christ, and we have for an example the disciples themselves. If anyone should have understood that Christ would die for the sins of man, it should have been Peter. John the Baptist is another example of the Holy Spirit ministering through men, as he proclaimed, "Behold the Lamb of God, which taketh away the sins of the world." Powerful statement, yet, while in prison, Joh sends his own disciples to ask of Christ...
Matthew 11
King James Version (KJV)
2 Now when John had heard in the prison the works of Christ, he sent two of his disciples,
3 And said unto him, Art thou he that should come, or do we look for another?
The Lord said of John...
Matthew 11:11
King James Version (KJV)
11 Verily I say unto you, Among them that are born of women there hath not risen a greater than John the Baptist: notwithstanding he that is least in the kingdom of heaven is greater than he.
John was not in the Kingdom of Heaven? Was John "saved?" He was "saved" in the sense that he belonged to God, yet, even being the second to last Old Testament Prophet, the revelation of the Mystery of Christ had not been made known to John. He inquired if Christ was in fact the Messiah. This indicates, I believe, or illustrates I should say, the ministry of the Holy Spirit in men, where they prophesy according to the Spirit yet have not full understanding revealed to them. Showing that the ministry of the Spirit of God did not begin with Pentecost, however, I think that given the revelation of the Mystery of Christ, which was kept hidden until the appointed time, we can see a distinct change in both the indwelling as well as that which He makes known unto us.
God bless.