It is also strange that Christianity would grow so much in the midst of such fierce opposition from Jews and Romans had Jesus been just a mythical figure invented by the apostles.
Leaving aside why, how much, when and how often were Christians (which ones?) persecuted... I fail to see how that proves or disproves the historicity of the main character of the Christian cult.
When a cult is suspected (rightly or wrongly) of threatening public order, the authorities are more concerned with its current practices and beliefs than with its historical origins. And many of the Eastern supertitiones which attracted a lot of people (and sometimes raised official suspicion) throughout the Roman empire were definitely built around mythical characters (e.g. Dionysos, Attis, Osiris, Mithra).
Also, the argument cuts both ways: it is equally strange that the Christ cult would grow so much and so quickly in the whole empire if it only started with the gradual deification of some historical Galilean character.
Not that I rule out this possibility: I personally find the issue undecidable. And secondary, inasmuch as a historical Jesus would hardly be more than the grain of sand over which the mythical Christ pearl was formed.