First, let's review what needs to be believed
What makes one a Christian is believing that Jesus saves us by virtue of:
- WHO Jesus is (answer: God incarnate)
- WHAT Jesus did (answer: offered his life for us)
- HOW To be united with Jesus (answer: through making Jesus our Lord in faith and through our public declaration during baptism)
- WHY being united with Jesus now impacts our own future after we die (answer: Read 1 Cor 15: Jesus's own resurrection was the first fruit of our own resurrection, v. 20; if Jesus's resurrection did not happen then our faith is in vain, v. 14)
Believing whether Jesus was resurrected historically is NOT the essence of being a Christian ALTHOUGH you should ask yourself HOW can a dead person saves you? So in a sense, yes, believing that Jesus was resurrected historically IS important, but you cannot stop at believing THAT the event happened. The essence of faith is trusting that Jesus who lives forever can now save you in the sense of the 4 points above.
Second, let's talk about epistemology: how to know whether you believe an event
How does one know whether one believes a historical event? Ask yourself: how do you know the date of your birth? You were still a baby at the time, you were not able to check the calendar; you have to trust your parents to tell you, or trust the government that your birth certificate is accurate. Therefore, to believe in a historical event necessarily includes believing in a trustworthy witness, which in this case, the trustworthiness of the Biblical witness of Jesus's historical resurrection.
But let's say you doubt the veracity of your birth certificate. Maybe it is fake, the real fact being that you were adopted: that your parents is not your biological parents, even though you have experienced your parents's love for 30 years. Does this doubt throw away your 30-year experience? Of course not. Trusting your parents's love based on your experience (of which you have good grounds, epistemologically speaking) is certainly MORE important than the validity of your birth certificate. In other words, believing the witness to an event is only the means to the goal: believing in Jesus who can save you NOW and the FUTURE.
Similarly, it's normal that Christians have doubt about some factual aspects of the Bible, i.e. how much of what the Bible claims as history really happened, etc. But what makes one a Christian is his/her experience with Jesus as God. Are you still willing to ask forgiveness from Jesus? If you are in mortal danger do you still believe that Jesus hears your prayer? If you are tempted to commit adultery do you respect Jesus's commandment more than following your desires?
If you answer all questions with a "yes", then you probably still has faith in Jesus. Blessed are those who have not seen and yet believe (John 20:29). Although your reason is not 100% convinced about the historicity of the resurrection, God has given you the grace to believe in the FACT that matters: the 4 points in the previous section. You are operating on the "light of faith" more than the insufficient "light of reason".
If you cannot answer all questions with a "yes", then it doesn't really matter whether you believe that the event happened or not. But if by asking these C.SE questions you have the desire to believe, ask God to give you the grace of the "light of faith" to believe the 4 points regardless whether your reason is fully convinced whether the historical event really happened or not. It's more important to believe that Jesus is alive NOW and in the FUTURE so He can save you.