An exquisite painting, on loan to the National Gallery of Art until Jan. 5, offers more than esthetic pleasure to the viewer. It marks a key moment in the historical process that ultimately was to give us a freedom we American Catholics cherish: our right to choose what faith we profess, to practice it openly, and to not pay taxes to support an established church.

The picture is the "Treaty of Muenster," by Dutch artist Gerard Ter Borch (1617-81). Highlight of the exhibition "Ter Borch," which gathers his refined portraits and upper-class genre scenes, the famous group portrait of 1648 shows delegates swearing to uphold the document that finally granted independence to the Protestant-ruled Dutch Republic from Spain, ruled by the Catholic Hapsburg monarchy.

On the left, the Dutch Calvinists raise two fingers in their pledge, while on the right the Spanish Catholic delegates place their hands on a Gospel book and crucifix. The scene is filled with portraits of the delegates who came to the north German town of Muenster, where the 30-year-old Ter Borch was hosted by the Spanish delegation.

Suspended over the proceedings is a very "Catholic" feature: a candelabrum crowned by a golden, aureoled statuette of the Virgin Mary of the Immaculate Conception, as the Woman "Clothed with the Sun" of the Apocalypse. (Fearful of Mariolatry, the Reformed Protestants excluded even Biblically based images of the Virgin.) Although he depicted the candelabrum (still existing) as a detail proving he had actually been there, Ter Borch made it much larger than it is in real life!

The Treaty of Muenster was one of the treaties collectively known as the Peace of Westphalia that ended the Thirty Years’ War. Sparked by strife between Catholics and Protestants, that war saw armies of unprecedented size lay waste to central Europe, the first "total war" in modern history. In the Peace of Westphalia, all parties agreed that a ruler could decide what religion would be established in his realm, but that Roman Catholics, Lutherans, and Reformed (Calvinist) could keep their churches no matter where they were.

The war and its outcome sharply advanced the separation of politics and religion in Europe. There were Catholic states on both sides, France against Spain. Pope Innocent X denounced the treaty, fearing a lessening of Catholic rights—but he was ignored. In the long run, papal spiritual authority was enhanced by the waning of papal temporal power.

The Peace of Westphalia did not offer the freedom of religion Americans now enjoy. But it did mean, for example, that Dutch Catholics—a large minority in the Protestant Netherlands—could practice their faith in private, enduring official discrimination, but not risking martyrdom.

Jan Vermeer (1632-75), the Dutch artist whose subjects and style were much influenced by Ter Borch, converted to Catholicism soon after the Peace of Westphalia; many of his paintings, such as the National Gallery’s own "Woman with Balance," show the veiled but deep impact of his Catholic faith.

"Ter Borch," on view till Jan. 5, is the latest in the series of National Gallery shows of leading Dutch and Flemish masters of the 17th century, organized by curator Arthur Wheelock, who gave the press a tour of the reopened Dutch galleries on Nov. 3. The permanent collection contains many treasures that can enhance our love for the Catholic traditions that persisted even under Calvinist rule.

In the newly acquired "Still Life with Swan and Game before a Country Estate," c. 1685, by Jan Weenix, Wheelock pointed out that the background is a stone relief of Virgin and Sleeping Christ Child, and a flying bird refers to the immortal soul.

In another case, the gallery has just purchased the stupendous "Interior of the Oude Kerk, Amsterdam," c. 1660-65, by Emmanuel de Witte (1616/17-92). This offers a great opportunity to contrast Catholic and Protestant modes of worship in the era of the Treaty of Muenster. De Witte was one of the two great specialists in church interiors, an important genre in the Dutch 17th century, rivaled by Pieter Saenredam whose picture of St. John’s Church in s’Hertogenbosch (1646) is hung directly opposite.

De Witte’s interior is typically Protestant, focused on the Word: the altar end of the church has been closed off, and attention is directed toward the pulpit from which the sermon was preached (all attention, that is, except for one feckless dog). Saenredam’s interior, in contrast, must have been painted for a Catholic patron. This church had in fact been taken over by the United Netherlands in 1629, and Reformed zealots had whitewashed the walls and stripped the "idolatrous" stained glass, statues, and pictures. However, in Saenredam’s rendering a colorful altarpiece of the Nativity has been imaginatively restored to the high altar, together with a statue of the Virgin and a tomb with the statue of the famously assertive local Catholic bishop kneeling toward the altar—all making the Eucharist the focus of worship.