Jesus' disciples followed Jesus for three and a half years. They also saw many miracles of Jesus. Nevertheless, they did not believe in Jesus as the Christ. (Lk 24:19)

Lk 24:19 And he said unto them, What things? And they said unto him, Concerning Jesus of Nazareth, which was a prophet mighty in deed and word before God and all the people:

Even though Peter confessed that Jesus was the Christ by the grace of the Holy Spirit, Peter himself did not believe in Jesus as the Christ. So he ended up denying Jesus three times. (Mt 26:73-74)

Mt 26:73 And after a while came unto him they that stood by, and said to Peter, Surely thou also art one of them; for thy speech bewrayeth thee. 74 Then began he to curse and to swear, saying, I know not the man. And immediately the cock crew. Eventually, all of Jesus' disciples ran away from Him. (Mk 14:50)

Mk 14:50 And they all forsook him, and fled.

Also, the disciples did not understand the Old Testament. (Lk 24:25-26, Jn 20:9)

Lk 24:25 Then he said unto them, O fools, and slow of heart to believe all that the prophets have spoken: 26 Ought not Christ to have suffered these things, and to enter into his glory?

Jn 20:9 For as yet they knew not the scripture, that he must rise again from the dead.

However, after Jesus was resurrected, He appeared to the disciples, and only then did the disciples truly believe that Jesus was the Christ. (Jn 20:27-28)

Jn 20:27 Then saith he to Thomas, Reach hither thy finger, and behold my hands; and reach hither thy hand, and thrust it into my side: and be not faithless, but believing. 28 And Thomas answered and said unto him, My Lord and my God.

The disciples believed that Jesus was the Christ, and only then did they understand the Old Testament and the words of Jesus. They came to understand that the Old Testament describes Christ and that Jesus also revealed Himself to be the Christ during His ministry. (Jn 2:22, Jn 12:12-16)

Jn 2:22 When therefore he was risen from the dead, his disciples remembered that he had said this unto them; and they believed the scripture, and the word which Jesus had said.

Jn 12:12 On the next day much people that were come to the feast, when they heard that Jesus was coming to Jerusalem, 13 Took branches of palm trees, and went forth to meet him, and cried, Hosanna: Blessed is the King of Israel that cometh in the name of the Lord. 14 And Jesus, when he had found a young ass, sat thereon; as it is written, 15 Fear not, daughter of Sion: behold, thy King cometh, sitting on an ass's colt. 16 These things understood not his disciples at the first: but when Jesus was glorified, then remembered they that these things were written of him, and that they had done these things unto him.

Peter, who believed that Jesus was the Christ, proclaimed that Jesus was the Christ even before death. (Acts 4:11-12)

Acts 4:11 This is the stone which was set at nought of you builders, which is become the head of the corner. 12 Neither is there salvation in any other: for there is none other name under heaven given among men, whereby we must be saved.

Even now, Jews do not understand the Old Testament because they do not believe that Jesus is the Christ. (2 Cor 3:14-16)

2 Cor 3:14 But their minds were blinded: for until this day remaineth the same vail untaken away in the reading of the old testament; which vail is done away in Christ. 15 But even unto this day, when Moses is read, the vail is upon their heart. 16 Nevertheless when it shall turn to the Lord, the vail shall be taken away.