"In my former book, Theophilus, I wrote about all that Jesus began to do and to teach"
Who was he?
Is it also possible that, since Theophilus means, "friend of God", as Richard states, that Acts is written to all believers?
Theophilus was the person addressed in both the Gospel of Luke and in the Acts of the Apostles (ie "Luke" and "Acts"). The identity of Theophilus is unknown. Per Wikipedia there are multiple possible options:
Unfortunately, we don't really know.
What we do know, however, is that Theophilus, in the original Greek, means "friend of God", "loved by God", or "loving God". This was a common name back then, but it was also an honorary title used among the academics of the day.
Wikipedia has much more info on the subject.