Of course, historians do differ about the earliest, formative period of Islam but there is little dispute that what we recognize today as orthodox Islam really standardized during the first few centuries of Islam. This is the period where you have the crystallization of Islamic law into four distinct schools which eventually encompass all the Muslim world (Hanafi, Maliki, Shafi'i, Hanbali). You also have different theological groups vie for power with the end result being three schools that recognize each other as at least having some claim to orthodoxy (Athari, Ash'ari, and Maturidi).

The standard text that defined orthodox Islam through history (and up till today) is entitled Aqeedah at-Tahawiyyah, the author of which died about 300 years after the rise of Islam.