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.
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). 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; John 16:27; John 17:8). Clearly Judas was not a believer, for Jesus calls him the son of perdition in his prayer in John 17:12. 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), Indeed, James did drink the cup and died a Martyr’s death (Acts 2:12). They were overzealous, perhaps sinfully so, but they still believed. As to Peter’s denial, we know from John 21 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). That passage in 2 Corinthians emphasizes that faith is the gift and work of the Holy Spirit (see also Ephesians 2:8). 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?
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