The Strive mechanic from Journey Into Nyx created some very interesting cards. Silence the Believers is an interesting piece of removal for Black. It's essentially an instant-speed version of Gild, exiling a target creature for four mana. However, while it doesn't give you the Gold "trophy" that Gild does, it does have the ability to exile additional creatures for 2B each.
While this sounds pricey, the ability to exile any Auras attached to said creatures can be pretty relevant in the right Standard format. At the time it was released, Silence the Believers seemed sure to see play with the rampant use of Bestow creatures. Bestow creatures that have been played as Auras would be exiled, as well, rather than hitting the board as a creature when the enchanted creature leaves the battlefield. At times, you're going to 2 for 1 or even 3 for 1 your opponent with this card. For 4 mana, that is pretty decent. The additional Strive cost is fair, as well.
Also, since it is exiling a card, Silence the Believers is able to take care of Indestructible creatures. During the Theros block, the most popular indestructible creatures were the Gods. And most of them were played a lot. Used correctly, it's pretty efficient removal, especially against indestructibles. While the Strive cost can be pricey, in mono-black, with Nykthos, Shrine to Nyx around, it could provide a lot of potential value printed onto one card.
What Really Happened with Silence the Believers in Standard?
Silence the Believers did see some amount of Standard play during the time of Theros block and Khans of Tarkir block, especially after the release of Magic Origins. The most successful deck to run Silence the Believers was Esper Dragons. Even then, it wasn't run in every Esper Dragons deck, and when it was, it was mostly a sideboard card. it was also occasionally seen in the mainboards of Abzan Midrange and Blue/Black (UB) Control.
Because of the metagame, it was worth running a copy. While the Gods weren't seeing as much Top 8 play at this point, there was one creature you really wanted to see exiled: Hangarback Walker. When the Walker died, he'd leave behind a bunch of flying Thopter tokens. Exiling the Walker means no tokens. It was also good against big creatures like the Dragons of the time and Siege Rhino. Four mana to deal with any one of these threats was worth it.
Silence the Believers Price History and Long Term Value
In most constructed formats, four mana is too much to remove any one creature. Typically, three mana is the most you'll see people want to pay, with cards such as Hero's Downfall and Murder fitting that bill. In Modern, 4 mana gets you Damnation. The format also has Path to Exile for a single White mana. So there isn't any room for Silence the Believers to succeed there, despite it probably being good in corner cases like against Aura-based Modern decks - Bogles, for instance. Usually, Path to Exile does the job, and Bestow and the Theros Gods (outside of maybe Keranos) see any Modern or Legacy/Vintage play.
How is Silence the Believers in EDH? Black is a very powerful color with plenty of mana acceleration available to it. Indeed, Silence the Believers does see a fair amount of EDH play, primarily in mono-Black decks. It's hardly a staple in the format, but as a card that can remove multiple creatures from play at once, it's certainly a playable removal spell. Exiling creatures is much more relevant in Commander than it typically is in other formats, but most Constructed formats usually have more efficient ways to do so.
Somewhat incredibly on Magic Online, Silence the Believers was a $5 card! It was actually worth more than $2 several other times during its Standard life. As it was a playable card in Standard, that's not totally crazy. In paper Magic, however, Silence the Believers was never worth more than $1.50, and that was at the release of Khans of Tarkir.
After its rotation from Standard, Silence the Believers' price hit about $0.25, but has slowly rebounded a penny at a time since then. This is mostly due to EDH playability. Still, you can easily find copies for a dime or two. While this card is hardly going to be a long-term sleeper, Silence the Believers could hit $1 in price someday, but it will be years before it does that.
Other than its EDH playability, the only way that Silence the Believers ever sees a major rise in price is if some Auras are printed that are poised to break the Modern format. While I wouldn't call Silence the Believers a good Magic "penny stock" to invest in, it can never hurt to stash a copy or two away of this bulk rare.