I once knew a sales manager in radio whose mantra was 'everyone listens to our station, therefore every business is a prospect'. The format was programmed to appeal to teens and he'd claim things like 'well, that's true, but their parents are driving them around and listening too and if you don't advertise with us, you'll miss those adults'. Needless to say it didn't take long for him to gain a reputation for trying to sell anything to anyone. No one questioned his passion, but neither did they believe his grandiose claims.
Let's take a look at passion from another perspective. What happens when sellers say they don't believe in something, or they believe one brand is superior to another?
First, if they're struggling from a moral standpoint and have ethical reservations, they need to quickly get out of that business or switch to another division. Nothing is ever worth compromising your personal values and integrity.
If what they really mean is that they don't believe in the value of the product, you have only one choice as I see it. Demonstrate the value and pricing structure. If you're unable to do this, that seller will struggle and likely make pricing concessions to close a sale.
If they don't personally like or find the product appealing, make it clear to them that you don't expect them to buy the product, you expect them to sell it. Big difference. It isn't about them and how they feel about the product, it's about whether the customer will benefit. Period. Never assume that the customer shares your preferences.
If, however, sellers are concerned about the level of support from other departments or they've been burned by fulfillment issues, etc, they will be hesitant to continue to sell with a sense of urgency. Should this be happening, get the help you need to uncover the cause(s) and make the necessary adjustments.
Although being passionate is good, selling will never be about sharing your passion for your product. In the end sales will always be about discovering a need and providing a solution.