Question:

Were the Apostles Christians prior to Pentecost? The betrayal of Jesus by Judas, the denial by Peter, and the desire of the "Sons of Thunder" to be at a special seating in Paradise might be examples that they fully did not understand what was happening before them until after the Resurrection.

Answer:

I believe your question to be an excellent one! Clearly there were no Christians, by name, until the people around them, starting in Antioch, called them Christians (Acts 11:26
²⁶And when he had found him, he brought him unto Antioch. And it came to pass, that a whole year they assembled themselves with the church, and taught much people. And the disciples were called Christians first in Antioch.

). Prior to that, and even prior to Christ, there were those who believed in Jesus and those who did not, and Jesus acknowledged this (John 6:69
⁶⁹And we believe and are sure that thou art that Christ, the Son of the living God.

; John 16:27
²⁷For the Father himself loveth you, because ye have loved me, and have believed that I came out from God.

; John 17:8
⁸For I have given unto them the words which thou gavest me; and they have received them, and have known surely that I came out from thee, and they have believed that thou didst send me.

). Clearly Judas was not a believer, for Jesus calls him the son of perdition in his prayer in John 17:12
¹²While 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.

. The Sons of Thunder clearly were ambitious, but it is not at all clear (a) that Jesus rejected their claim to love him, or (b) that they understood what they were asking for (Matthew 20:23
²³And he saith unto them, Ye shall drink indeed of my cup, and be baptized with the baptism that I am baptized with: but to sit on my right hand, and on my left, is not mine to give, but it shall be given to them for whom it is prepared of my Father.

), Indeed, James did drink the cup and died a Martyr’s death (Acts 2:12
¹²And he killed James the brother of John with the sword.

). They were overzealous, perhaps sinfully so, but they still believed. As to Peter’s denial, we know from John 21
¹⁴This is now the third time that Jesus shewed himself to his disciples, after that he was risen from the dead.
¹⁵So when they had dined, Jesus saith to Simon Peter, Simon, son of Jonas, lovest thou me more than these? He saith unto him, Yea, Lord; thou knowest that I love thee. He saith unto him, Feed my lambs.
¹⁶He saith to him again the second time, Simon, son of Jonas, lovest thou me? He saith unto him, Yea, Lord; thou knowest that I love thee. He saith unto him, Feed my sheep.
¹⁷He saith unto him the third time, Simon, son of Jonas, lovest thou me? Peter was grieved because he said unto him the third time, Lovest thou me? And he said unto him, Lord, thou knowest all things; thou knowest that I love thee. Jesus saith unto him, Feed my sheep.

that the Lord did not accuse him of unbelief but rather restored him to the position of friend that he had enjoyed before denying him.

Your last statement, that they did not fully understand until after the resurrection is a key point. We must not equate faith with understanding. Long ago a believer named Anselm put it this way: I believe in order that I may understand. Indeed, there is a veil over the eyes of the Jews so that they cannot understand the Scriptures - a veil tied to their unbelief (2 Corinthians 3:12-18
¹²Seeing then that we have such hope, we use great plainness of speech:
¹³And not as Moses, which put a veil over his face, that the children of Israel could not stedfastly look to the end of that which is abolished:
¹⁴But 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.
¹⁵But even unto this day, when Moses is read, the vail is upon their heart.
¹⁶Nevertheless when it shall turn to the Lord, the vail shall be taken away.
¹⁷Now the Lord is that Spirit: and where the Spirit of the Lord is, there is liberty.
¹⁸But we all, with open face beholding as in a glass the glory of the Lord, are changed into the same image from glory to glory, even as by the Spirit of the Lord.

). That passage in 2 Corinthians emphasizes that faith is the gift and work of the Holy Spirit (see also Ephesians 2:8
⁸For by grace are ye saved through faith; and that not of yourselves: it is the gift of God:

). Do not tie understanding to faith so tightly as to require full understanding before you can be saved! I do not understand gravity, but I believe it exists! I do not understand the Trinity completely but I do believe God is one God in three persons. I do not understand how God can love a sinful man like me, but thank God he does not require it but only calls me to believe that he does because Christ has saved me by his life and death. Have you trusted in Jesus?