The quote is correctly worded:
Les passions sont les seuls orateurs qui persuadent toujours.
The passions are the only orators which always persuade.
This is how the quote appears in Kay Redfield Jamison’s Exuberance: The Passion For Life, Random House, 2004:
“Passions are the only orators which always persuade. They are like an act of nature, the rules of which are infallible; and the simplest man who has some passion persuades better than the most eloquent who has none.”
Kay Redfield Jamison, John Hopkins University psychologist and noted author, passion is often a component or byproduct of "exuberance," a heightened biochemical and expansive psychological state. In her recent book, Exuberance: The Passion for Life (Alfred A. Knopf, 2004), Jamison observes that, "Passion rides roughshod over hesitating judgment; it dissolves inhibitions. It provokes play and exploration of another's work…Passions bring to our attention the overlooked; they compel commitment of time and heart. They persuade by sheer dominance of the emotional and mental field…However, without a counterweight or discipline it can be dazzling scattershot: excitement without substance, all fizz and no gin. When (high spirit and unrelenting optimism) lacks a fuller emotional or intellectual context, it can become intrinsically shallow…lacking the gravitas of the tragic or heroic…not struggling with profound issues of humanity, not contending with the shadows cast by death. Exuberance is not an inward-looking state; it looks upward and forward, rarely to the past. Disquieting emotions are overpowered by excitement of the idea; the past cedes territory to the present and future."http://www.mhcinc.org/poc/view_doc.php? ... 621&cn=116