This is an instruction to foreigners of all nations: Foreigners who trade in Canton have realized large profits. They can sell all the goods they have brought to China and purchase on short order any merchandise they wish to buy. Because of this fact, the number of ships that come to China to trade has increased from 50 or 60 in the old days to more than 150 in recent years. His Majesty the Emperor allows all of you to trade in China without discrimination, and his generosity has provided you with the opportunity to realize the profit you desire. If the trade is stopped, where will your profit come from? Feeling grateful for the favor you have received, you should at least observe our law and refrain from enriching yourselves by deliberately inflicting harm upon your benefactors. Why do you choose to ship to China opium which you yourselves do not consume in order not only to swindle people out of their money but also to endanger their very lives? You have used this evil thing to poison the Chinese people for dozens of years, and the amount of profit you have realized from this immoral trade must be very large indeed. This devilish conduct on your part not only stirs the indignation of mankind but is intolerable to Heaven as well . . .

give me the main idea in 1 sentence

The main idea of this passage is a plea to foreigners to stop the immoral and harmful opium trade in China, emphasizing the generosity and opportunity provided by the Emperor for profitable trade.