They might have been too busy evangelizing and getting martyred to write history books to most of the people who couldn't read anyway.
We know something big was happening for there to be such dynamic growth. Luke reports Peter converted 3000 one day and 5000 men (not including women) shortly afterward. Could there have been more going on then just the formerly cowardly Peter's preaching?
What do you have in mind? Preaching can convert large numbers of people. Even false preaching. People get martyred for false, as well as true, ideas. Anyway I simply don't believe the figures you give. Cite your source, please. The growth of Christianity was pitifully slow. So slow that decades passed before anyone recorded its existence. The first Roman historians to do so wrote around 110-120 AD. At that time the Roman politician Pliny knew almost nothing about them, and had to ask the Emperor Trajan's advice on what to do with them. About the same time, the historian Tacitus had to explain to his readers who the Christians were, when he mentioned them in a book.
http://www.abc-of-christianity.com/info/first-five-centuries.asp
Some historians estimate that about 300 A.D. there were about 50 million people in the Roman Empire, of whom about 10 million were Christians.
So they were still a small minority more than 250 years after Jesus' death. For the incidence of persecution, see
http://wiki.answers.com/Q/How_were_the_early_Christians_persecuted in particular the famous statement by Origen. Persecution appears to have been sporadic up to the late third or early fourth centuries. Most (not all) of the stories of early martyrs are preposterous legends. Philomena, Eulalia, Agnes, Perpetua - nonsensical stories. Look them up. We know nothing about the fate of Paul. Persecution of Jews under Christianity was far more severe. Does that make Judaism true?
Or is Islam true? Its growth was vastly more "dynamic". Muhammad died in 632. By exactly 100 years later the Muslims ruled Arabia, Syria, Persia, Egypt, the rest of North Africa and Spain, and had invaded France, where their expansion was finally halted by a defeat at Tours, on the River Loire. A hundred years after Jesus' death, Christianity was only beginning to be noticed, as a band of superstitious sectarians, by the commentators of the day. So your argument is a very unconvincing one, and it would be best left to Muslims, who indeed use it.
In any case, it's a false argument. Truths are not established by majority vote. Or even by "dynamic growth". Aristarchus of Samos, who died about 230 BC, correctly suggested that the Earth goes round the Sun. Almost nobody accepted this, and about 1,850 years later people were still being punished for suggesting it. Does that slow non-dynamic progress mean the idea is wrong?