Most people use common sense when reading and writing. Obviously, a bald statement about the way things should be is a statement of opinion—unless you are religious and believe the statement has come from God Himself. Thus, to a Christian, the commandment "Thou shalt not kill" is not just God's opinion.
I spoke of "a bald statement of fact" above because human beings are rational and engage in something called argument, wherein reasons are proffered in support of statements in an effort to show that those statements truly reflect the way things are and are not just the opinions of the speaker or writer.
From a grammatical standpoint, you can introduce a series of mere opinions by using, for example, "He thinks/believes/says . . ." followed by a series of "that"-clause complements, subordinated to that matrix verb of opinion. If it is unclear whose statement it is, the reader may assume it is your opinion.
Another way you can introduce a series of opinions with only one clause or phrase of attribution is by using "According to So-and-So. . ." I mentioned this above, but you were so busy repeating your question that you may not have noticed it.
If the person has sought to set his or her opinion above the status of mere opinion by using argumentation, and you feel that reasons that he or she has given for the statement are good enough to raise the opinion into the realm of possible fact, then you say, "So-and-So explains that . . . ."
Even if the argument is bad, you can still say, "So-and-So argues that . . . ."