“Muslim merchants traveled outside the empire to many new lands. They carried their faith with them. Missionaries spreading Islam often accompanied traders bring their faith. Over time, many countries of South Asia, Southeast Asia and Philippines turned to Islam.”  p.461 My World History (Early Ages)

From the textbook, the student is left with the assumption that trade was a major reason for the spread of Islam. I found this theory in other textbooks.

Not true. Yes, it is true that trade always follows the conquest of other lands, and Islam was no exception, but the trade followed the conquest. And while we are speaking about trade, there is no mention of the slave trade in the textbook. Slave trading was the first item on the trading list after a country was conquered by Islam. And free trade was the reason Christopher Columbus sailed east to find India and China. Muslims had  blocked freedom of sea trade through the Mediterranean Sea. Spain had to go east to avoid the Muslim pirates of the Mediterranean.