Just believing that God was going to send a Messiah, a person anointed to save Israel and all mankind, does not make you a Christian and part of the Church (body of Christ). All the Jews believed that, but they didn't all become Christians.
It's a complicated few verses! Who are the "prophets" that Peter was referring to? It could mean the Old Testament prophets, but it could also be referring to those in the Church who had the gift of prophecy. As Barnes comments:
... it remains to give that which seems, on the whole, to be truer to the meaning of the words, and which presents a solution of phenomena which the other leaves unsolved. The basis of this other explanation lies in the belief that St Peter is speaking mainly, though perhaps not exclusively, of the prophets of the Apostolic Church. The position of those prophets was, we must remember, as prominent as that of the Apostles (Eph_2:20; Eph_3:5; Eph_4:11; 2Pe_3:2). Among those with whom St Peter had been brought into personal contact were Barnabas, the “son of consolation,” or, as the Hebrew might be interpreted, the “son of prophecy” (Act_4:36), Agabus (Act_11:28; Act_21:10), Judas, and Silas or Silvanus (Act_15:32). In 2Pe_1:19 we have sufficient proof of the importance attached to the “prophetic word” as a light giving guidance amidst the darkness and perplexities of the time. In 2Pe_3:1-13 we see that they spoke of the glories of the new heaven and the new earth after a time of darkness and distress. In 1Co_2:9-10 we read how the things which “eye had not seen nor ear heard” had been revealed to prophets by the Spirit, and in Rom_16:25-26, in like manner, that “the mystery which had been kept secret since the world began was now made manifest in prophetic writings,” just as in Eph_3:5 St Paul speaks of the same mystery as now “revealed unto the Apostles and Prophets by the Spirit.” All this is enough, it is believed, to warrant, if only at first, tentatively, the assumption that the prophets of the New Testament are those of whom St Peter speaks.
Verse 12 says, "To them it was revealed, that not to themselves, but to you, they ministered these things, which now have been announced to you through those who preached the Good News to you by the Holy Spirit sent out from heaven", meaning that the prophecy that came to the prophets was not just for them but was to be shared with all Christians, and was shared by the Apostles and Christian teachers/evangelists. Peter was saying that when the Spirit of Christ working in the prophets predicted the sufferings of Christ (and Paul said "the sufferings of Christ abound to us", 2Cor 1:5) and the glories that would follow them, then, as Barnes explains it, "they perceived that in their communications there were some great and glorious truths which they did not fully comprehend, and that they diligently employed their natural faculties to understand that which they were appointed to impart to succeeding generations. They thus became students and interpreters for themselves of their own predictions".
Even if Peter was referring to the Old Testament prophets, just because they recorded prophecies that pointed to the Messiah, such as Psalms 22 and Isaiah 53, that does not make them Christians. To be a Christian you must be called by God and begotten again:
Ephesians 1:13 (ESV):
(13) In him you also, when you heard the word of truth, the gospel of your salvation, and believed in him, were sealed with the promised Holy Spirit,
2Thessalonians 2:13-15 (ESV):
(13) But we ought always to give thanks to God for you, brothers beloved by the Lord, because God chose you as the firstfruits to be saved, through sanctification by the Spirit and belief in the truth.
(14) To this he called you through our gospel, so that you may obtain the glory of our Lord Jesus Christ.
(15) So then, brothers, stand firm and hold to the traditions that you were taught by us, either by our spoken word or by our letter.
Those living before the establishment of the Church did not hear or read the Gospel and so couldn't believe it or have a part in its blessings.
The "them" referred to was not Old Testament saints. It was to the Israelites that Moses brought out of Egypt and into the wilderness. Of them it says:
Hebrews 3:9-11 (WEB):
(9) where your fathers tested me by proving me, and saw my deeds for forty years.
(10) Therefore I was displeased with that generation, and said, ‘They always err in their heart, but they didn’t know my ways;’
(11) as I swore in my wrath, ‘They will not enter into my rest.’”
Hebrews 3:19 (WEB):
(19) We see that they were not able to enter in because of unbelief.
So Paul then writes (to quote the whole verse):
Hebrews 4:2 (WEB):
(2) For indeed we have had good news preached to us, even as they also did, but the word they heard didn’t profit them, because it wasn’t mixed with faith by those who heard.
So those Israelites failed to profit by the good news of their release from captivity in Egypt and of the promised land and blessings of God, because they didn't believe God. Therefore God did not let them enter the promised land and they all died in the wilderness. The main point of Paul's writing is verse 11 (WEB):
(11) Let’s therefore give diligence to enter into that rest, lest anyone fall after the same example of disobedience.
As you should know, it has been revealed to the Church that there are two resurrections. The Jews believed in a resurrection, but they didn't know of the first resurrection of the Church. Paul reveals things that were not revealed to the Old Testament Israelites. Colossians 1 (WEB):
(26) the mystery which has been hidden for ages and generations. But now it has been revealed to his saints,
(27) to whom God was pleased to make known what are the riches of the glory of this mystery among the Gentiles, which is Christ in you, the hope of glory;
