One of the themes of the NT narrative is the idea that Christ arrived on the scene just at the right time. He wasn't too early, he wasn't late. Instead, he came when the world was ready, even ripe, for the truth he brought. This idea can be found in the NT in passages such as Mark 1:15, Gal 4:4, Eph 1:10, 1 Tim 2:6, and Titus 1:3.
When one studies the historical context surrounding the entrance of Christ, we find indeed, the best possible time for the entrance of the incarnated Son of God was around AD 3-4.
There are 3 main reasons for this.
The World was Prepared Politically
Since the "known world" was conquered by the Romans, the world was politically stable enough to receive Christ. The Romans had enforced a peace enabling Jesus to carry out his ministry, and subsequently the peace also enabled the spread of the Gospel.
Many people had immigrated to the cities, where ideas were being exchanged freely in the melting pot of the cultures. This meant Jesus would be heard in the cities as another voice. Thus many nations would quickly hear about the teachings of Christ.
Not to be overlooked, the Romans also established an extensive system of roads. These roads made it possible for easier travel between cities. It also helped the authorities enforce laws and thus reduce thievery of travelers.
All in all Rome created a safe haven for the spread of the Gospel.
The World was Prepared Intellectually
When Alexander the Great had conquered the "world" he did something very beneficial to the ministry of Christ. Having died in 323 B.C. Alexander left the world more "greek" than it was before he had conquered it. One of his goals was to make the world "greek," and as such, he had soldiers marry native women, and teach all of the people the Greek language. Also, the Old Testament was translated into the Greek by 280 B.C. As such, when Jesus arrived the people had a common language to communicate their ideas. Jesus probably spoke Aramaic most often, but a common language meant his life and ideas could be communicated easily.
Philosophically, the world was also ready for the person of Christ. Greek philosophy had taught a few positive things which helped whet the people's appetite for Jesus of Nazareth. Socrates taught about morality, Plato taught about a soul and an immaterial realm, and Aristotle argued for the existence of a "prime mover" or God. All of these things impacted the world positively for Christ.
On the other hand, philosophy had not met the needs the people had. Regular Joes did not find fulfillment in philosophy, it was a system for the elite, strong, and affluent. It did not make any promises to people who understood their sin, and were looking for love and redemption.
Lastly, the Greeks had also shown the world that not every religion would cut it. The religion of the Greeks and Romans had failed the people in so many ways. They offered a polytheism with vengeful, spiteful, egotistical gods that seemed to rather hurt people than help them. If one of these gods said they loved you, you would cower or run for fear of rape (Zeus was especially known for his lust for mortal women)! These gods did not meet man's needs. Instead this type of religion made the masses hunger for something much more satisfying to their needs and intellectual capacities.
The World was Prepared Religiously
The Jews had sufficiently kept the ideas of Judaism in a way as to be able to share them with the world. They had preserved monotheism. They had given the world the Old Testament in the common language of the time, and had developed an extensive system of worship throughout the known world. This led to many gentiles embracing Judaism in the synagogues. The influence of these synagogues was important in preparing the world for Christ.
Christ's Impeccable Timing
Jesus came to this earth when just the right circumstances came together to maximize his impact on this world, thus changing the face of Western Civilization, and consequently the whole world. His life ignited a world-wide movement, spreading like wild-fire. We need to continue preaching the Gospel, waiting for his perfect timing when he comes again.