This is my oversimplification of the identity and context of the Kirchenverein, later the (German) Evangelical Synod of North America, explained using two charts.
The definitive history is in The German Church on the American Frontier, the PhD dissertation of Carl Schneider, professor at Eden.
King Frederick William III of Prussia was an absolute monarch who wanted an end to the Lutheran-Reformed divide. This may have been informed by his inability to take communion alongside his Lutheran wife.
The King did not disregard the differences by imposing one confession over the other, or by crafting an all-new union confession to displace the two former sides. The sides weren't in fact eliminated, they were just made to cooperate. Frederick William personally researched the development of the liturgy and began to roll out a unified agenda for churches in Prussia. This unity in worship also came with a unity of structure.
The new common identity was simply "Evangelical" and the church the "Evangelical Union." The fiercest opposition came from the Lutherans, and those who stayed out were called the Old Lutherans. Atheistic rationalists were outside of the church altogether, while Pietists may have been either Lutheran or Reformed.
All of these parties were represented in migration to America.
German-speaking Lutherans and Reformed Christians had been in North America since the colonial era. Unavoidably, successive generations began the process of Americanization, distancing their children from the practices of their Old World cousins. They also found themselves insulated from the King's Evangelical Union experiment.
Frontier immigrant settlers detected the cultural distance, and different types of immigrants each carried with him a type of religious ideal.
Rationalists were liberals who escaped the monarchical rule of their native lands and hoped to establish atheistic communities in the New World (as in Comfort, Texas). They viewed their evangelical neighbors as enemies of their experiment.
Much of German frontier immigration was focused on the so-called "Missouri Rhineland." The Saxon Migration was composed of "Old Lutherans" who opposed compromise with Reformed theology, and felt burned by their old state's perseuction of their traditional beliefs. They also were not fond of the Evangelicals. These immigrants formed the Missouri Synod.
The evangelicals walked a tightrope. Their churches may have been essentially Reformed or Lutheran. They had a Pietist influence that led to a greater emphasis on practice than scholastic orthodoxy. For some, they may have just been tired of the old world fight that they just left, or, unionism was all they knew. Still, they deliberately avoided affiliating with the Reformed synods until merger in 1934.
When the Evangelicals found their footing, they first set about crafting a catechism. They looked to an existing union catechism called the "Unterbarmer Katechismus," then abandoned it and started from scratch. The result was the 1847 Evangelical Catechism, which in 1862 was reduced from 219 to 137 questions by Andreas Irion, Marthasville Seminary President, and titled the Kleiner (smaller) Evangelical Catechism. It was last revised in 1929.
In a sense, the Synod forgot that they had a long catechism. The shorter catechism became their only version. The opposite happened in the Reformed church - they had only the long Heidelberg, and kept reinventing the shorter catechism through the years.
The Evangelical Catechism follows Luther's catechism in structure, not Heidelberg, but uses the Reformed system of numbering the commandments. Conversely, it follows Luther more than Heidelberg on Christ's descent in to Hell. It is direct and biblical more than precise and technical.
Is there a place for the Evangelical Catechism today? Walter Brueggemann revisited it in 1972, well after the UCC merger. His read is uneasy, mixing admiration with embarrassment, unsubtly advocating for a move away from the use of the catechism as intended, or even rejecting the premise of historical catechesis in the 21st century. His insights are still helpful, especially in noting the slight liberationist intention in the 1929 revision. Pedagogically, he wished instead for an analogue to the experimental Catholic Dutch Catechism.
One must agree with Brueggemann that the catechism hardly had a place with his 20th century Protestant Liberalism. The catechism therefore belongs to the Evangelicals - in the political sense of that word. 21st century American Evangelicals share the same presuppositions with the old Evangelicals: that the Bible is God's inspired & authoritative word to man, that right doctrine can be known, that a "knower-seeker" relationship exists between ministers and their students. Pietism, it is often argued, even shows its pervasive influence throughout Evangelicalism.
The catechism is ripe for recapture, and it has some publicly identifiable use today. The First Protestant New Braunfels, a founding church of the Evangelical Association, has formally adopted a revised 1929 edition. The Evangelical Association itself publishes a modernized 1896 edition as the "Traditional Orthodox Edition."