Historically, how did Protestantism first arrive at a consensus? Paul urges the Corinthian church :


  Now I beseech you, brethren, by the name of our Lord Jesus Christ, that ye all speak the same thing, and that there be no divisions among you; but that ye be perfectly joined together in the same mind and in the same judgment. I Corinthians 1:10 KJV.


In order for Protestantism to exist at all, initially, there had to be a consensus among some regarding the Reformation, and, particularly, regarding doctrine.

How was that consensus arrived at, historically, and what lessons does this have for the present situation of Protestantism being divided into contradictory denominations?

How did the original Reformers arrive at agreements of doctrine and how could this be reiterated now, or in the near future?
The Reformation beginning in the 16th century was actually three different movements consisting of: (a) Calvinism (b) Lutheranism (c) Anabaptists.  There were conferences between these groups but the only things they were united upon were:


Opposition to the authority of the Catholic church largely due to its excesses and abuses
Salvation by faith alone (the five "solas" actually came later)
Doctrine based on Scripture


However, there were numerous fights between these groups.  For example, Calvin persecuted Lutherans and Anabaptists by having some executed, etc.  Conferences between Luther and Zwingli ended in stalemates.  There were on-going debates particularly about how communion should be practiced and what it meant, which persist today.

As time progressed, further subdivisions emerged such as "enthusiasm" (the basis of the charismatic movement) and many more.  Thus, the reformation was never truly united.

Good overall summary: https://en.wikipedia.org/wiki/Reformation as well as https://en.wikipedia.org/wiki/Anabaptism