When did William I join the Reformed Church?

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Portrait of William I, Prince of Orange, by Adriaen Thomasz Key. He wears a dark embroidered coat, a white ruff collar, and a black cap, with a trimmed beard and mustache. — William I Portrait of William I, oil on panel by Adriaen Thomaszoon Key, c. 1579; in the Rijksmuseum, Amsterdam.

William I, the Prince of Orange, officially joined the Reformed Church in 1573.1

While he had previously reverted to the creed of his childhood and developed a close working relationship with Calvinists as the core of the resistance movement against Spanish rule, his formal affiliation with the church did not occur until that year.1