Were the oldest Buddhists evangelical?

There was no knocking on doors tradition.It was an era of wisdom and everyone was so exited to see who is right.So when a sage comes to a village people will go to see him as a habit.If they accept what has been said they continue to be followers or they look away.

This excitement was not very far from what we see today.I am sure that you have seen how people wait in line to buy the new "Apple iPhone" or how they wait to see the next "James Bond" movie.

You might say it was diferent but it was actually quite a happening time. For the first time they saw human defy gravity,walk in water,levitate,teleport.

They were exited to see who is right / who will win this battle of beliefs. A good example can be found in "Maha mangala sutta".In it a being from another realm (a Deva) says that there is a certain struggle among humans that even found its way to their realm.

The question was "What is mangala / what is good or fortunate"

This was the best time for religions because it wasn't a time like this in which people are too busy for spiritual things.People were actually waiting to see what will happen next.There was a cultural and spiritual awakening in the society of India.

And Lord Buddha had no trouble introducing himself after all he was a prince once with a huge and powerful kingdom.Lord Buddha gave up all of that to find the path.Even before the enlightenment Prince siddhartha (The prince who became lord Buddha) was quite like a celebrity among the people.

But there was a special method to Lord Buddha.

Once lord Buddha saw a future of a farmer.That man was about to loose everything to a flood.But lord Buddha did not said anything,instead Lord Buddha visited him in his paddy field daily and talk to him.It was only after the flood Lord Buddha taught him dhamma.

Almost all of other religions came to debate with lord Buddha to ensure their survival against this new path.All of them were lost,and the major supporters of these beliefs became Lord Buddha's followers.This was a huge advantage.Not to mention all the sages who lost and became followers themselves like the "3 Jatila sage leaders Gaya kashyapa,Nadee kashyapa and their brother"

"Go forth, o bhikkhus, for the good of the many, for the happiness of the many, out of compassion for the world, for the benefit, for the good, for the happiness of gods and men. Let not two go by one way. Preach the doctrine that is beautiful in its beginning, beautiful in its middle, and beautiful in its ending. Declare the holy life in its purity, completely both in the spirit and the letter." ~ Mahavagga, Vinaya Pitaka."

I'd imagine it spread much the same way as other religions. In many cultural centers of the ancient world you would have places where multiple beliefs congregated to talk to those who walked by about their faith.

Imagine going to the mall today and in center court you had a Christian, Hindu, Buddhist, Jewish, Muslim, and taoist all speaking of their doctrines?

You get a glimpse in the Kalama sutta of the time of the Buddha being one where teachers and traditions were numerous and they wandered about teaching their doctrines.

"As they sat there, the Kalamas of Kesaputta said to the Blessed One, "Lord, there are some brahmans & contemplatives who come to Kesaputta. They expound & glorify their own doctrines, but as for the doctrines of others, they deprecate them, revile them, show contempt for them, & disparage them. And then other brahmans & contemplatives come to Kesaputta. They expound & glorify their own doctrines, but as for the doctrines of others, they deprecate them, revile them, show contempt for them, & disparage them. They leave us absolutely uncertain & in doubt: Which of these venerable brahmans & contemplatives are speaking the truth, and which ones are lying?"

So it is obvious that Buddhism was spread much the same way as other traditions in ancient times. What makes Buddhism mostly different is that there are almost no examples of buddhism involving forced conversation.

There is no doubt that king Asoka did for Buddhism what King Constantine did for Christianity hundreds of years later. Once it became a religion of the dominant Kingdom its spread was assured as it went from one of many doctrines to the official state doctrine.

Adding to Jayantha's very good answer:

One needs to look at Indian society at the time of the Buddha. There were many masters profounding various siddhāntas(established and accepted view of a particular philosophical school). It is called an 'established conclusion' but that view is final and definitive for the one who has settled on it. It does not necessarily mean that the person will never abandon that view, but for now that person thinks it is final.

The masters continued transmission of their siddhāntas through paramparās(lineage of followers). These masters had great respect among people and many went to different masters seeking guidance and became a follower of a siddhānta. Often, one paramparā was challenged by another paramparā with philosophical arguments. The victor persisted. Thus we could consider that at that time, every master was a missionary. This answers your question if all early Buddhists were missionaries.

But there is a difference in the kind of missionary religion Buddhism is. As Dr. Arvind Sharma puts it:

"Buddhism is indeed a missionary religion, it possesses a feature which distinguishes it from Christianity and Islam as missionary religions. When one converts to these two religions, one is supposed to break from one’s previous religious past and then substitute Christian or Islamic beliefs and practices for the old ones, which are now abandoned. Buddhism, however, does not insist on such a break and conversion to it need not entail the abandonment of one’s religious past. In other words, we have another dichotomy here, between those religions which require one to abandon one’s previous religious loyalties upon conversion, and those which do not."^‡

References:

• ‡ Sharma, Arvind. "Of Conversion and Reversion." OPEN Magazine. N.p., n.d. Web. 09 Nov. 2015.

• Perdue, Daniel. "Reasoning within the Buddhist Context." The Course in Buddhist Reasoning and Debate: An Asian Approach to Analytical Thinking Drawn from Indian and Tibetan Sources. N.p.: n.p., n.d. N. pag. Print.