Religion in the United Provinces was a complex thing that evolved over time. William of Orange only lived until the very beginning of the United Provinces as an independent state. He died before the Dutch state had fully crystallized so what he personally felt, as well as in what direction he wanted to take the United Provinces is not the same as how the Dutch state eventually ended up handling religious toleration.

As for William of Orange himself, he can indeed be generally construed as supporter of religious toleration. But in an age of religious turmoil in Western Europe concerning the reformation, extremism and hostility abounded, and taking a stance of toleration was not an easy thing to do amidst all this religious infighting. William of Orange was nicknamed 'William the Silent' because he rarely spoke his mind and kept his personal convictions close to his chest. He was a charismatic, politically talented leader who worked to unite disparate factions under a common goal (resistance to Spanish tyranny). He handled religion in this way, allying himself with protestant rebels while at the same time trying to protect Catholics from the fanatical protestants who sought to wipe them out. He tried to take the middle-way.

Also,

was that the United Provinces were generally tolerant, with multiple religious denominations co-existing.

This is not exactly true. The United Provinces were officially a protestant state. The government officially supported a protestant state church. However, the United Provinces were a very decentralized state, and its component provinces and cities had a very very large degree of self-rule, including decisions relating to religion. The only thing they really agreed upon is that Catholicism was not allowed to be practiced publicly. However, while the government was officially protestant, the Dutch population was very divided, including a huge minority of Catholics. To prevent this religious divide from tearing the country apart, policies of toleration were a necessity. In practice, Catholicism was tolerated in many places provided that they practiced their faith privately, and not openly. But because the United Provinces were so decentralized, a blanket statement of toleration cannot be made because there were also a lot of places that did not tolerate Catholics.

Judaism, as well as minority protestant streams were treated similarly, though the places where Judaism was tolerated were a much smaller number (they were basically confined to a few cities).

So to sum it up, the United Provinces officially supported one particular denomination of Protestantism, but a number of other Protestant denominations as well as Catholicism, and Judaism, were allowed to exist in certain parts of the state, with varying degrees of freedom, the general rule being that they had to keep their religious practices private and away from the public eye.

Are you asking about William the Silent or William III? There are answers here about both so far.