what does it mean to be a Christian?

It means to turn your life over to follow and be an obedient servant of Jesus
Christ.  (Contrary versions are "I am a good Christian, but I don't go to
church."  That is a contradiction.  Going to church does not mean you are a
Christian.  But not going to church means you can't be a good Christian.
Because all Christians participate in the living body of Christ where 2 or 3
gather to worship, to serve, and to forgive.  There is an act of repentance
or change and turning around of one's life first for anyone who is a good
Christian which means you are on a journey of letting go of your life, dying,
to rise with Christ.  Unfortunately, our secular culture has captured or
hijacked the meaning of being a "good Christian" and turned it into a secular
meaning e.g. belonging as a member to an institution, or being a good person.
Neither one of these are at the heart of being a Christian.  And these must
have been issues back in the 1st Century because Jesus and Paul both confront
these notions of being a "good Christian" in scripture.  Which then raises
the issue of how much have people who think they are "good Christians" have
not studied the Bible.