The Heidelberg Catechism, one of the Three Forms of Unity, is a Protestant confessional document taking the form of a series of questions and answers, for use in teaching Reformed Christian doctrine. It has been translated into many languages and is regarded as one of the most influential of the Reformed catechisms.
Elector Frederick III, sovereign of the Electoral Palatinate from 1559 to 1576, commissioned the composition of a new Catechism for his territory. While the catechism's introduction credits the "entire theological faculty here" (at the University of Heidelberg) and "all the superintendents and prominent servants of the church" for the composition of the catechism, Zacharius Ursinus is commonly regarded as the catechism's principal author. Caspar Olevianus (1536–1587) was formerly asserted as a co-author of the document, though this theory has been largely discarded by modern scholarship. Frederick wanted to even out the religious situation of the territory, but also to draw up a statement of belief that would combine the best of Lutheran and Reformed wisdom and could instruct ordinary people on the basics of the newfound Protestant version of the Christian faith. One of the aims of the catechism was to counteract the teachings of the Roman Catholic Church, and so it based each of its statements on the text of the Bible.
Commissioned by the sovereign of Palatinate, it is sometimes referred to as the Palatinate Catechism.
The Catechism is divided into fifty-two sections, called "Lord's Days," which were designed to be taught on each of the 52 Sundays of the year. The Synod of Heidelberg approved the catechism in 1563. In the Netherlands, the Catechism was approved by the Synods of Wesel (1568), Emden (1571), Dort (1578), the Hague (1586), as well as the great Synod of Dort of 1618-1619, which adopted it as one of the Three Forms of Unity, together with the Belgic Confession and the Canons of Dort. Elders and deacons were required to subscribe and adhere to it, and ministers were required to preach on a section of the Catechism each Sunday so as to increase the often poor theological knowledge of the church members. In many Dutch Reformed denominations this practice is still continued.
In its current form, the Heidelberg Catechism consists of 129 questions and answers. These are divided into three main parts:
I. The Misery of Man 
This part consists of the Lord's Day 2, 3, and 4. It discusses:
II. The Redemption (or Deliverance) of Man 
This part consists of Lord's Day 5 through to Lord's Day 31. It discusses:
- The need for a Redeemer
- The importance of faith, the content of which is explained by an exposition of the 12 Articles of the Christian faith, known as the Apostle's Creed. The discussion of these articles is further divided into sections on:
- God the Father and our creation (Lord's Days 9-10)
- God the Son and our salvation (Lord's Days 11-19)
- God the Holy Spirit and our sanctification (Lord's Days 20 - 22)
- The Sacraments of Baptism and the Lord's Supper
- And the keys of the kingdom of heaven The Preaching of the Gospel and Church Discipline
III. The Gratitude Due from Man (for such a deliverance) 
This part consists of the Lord's Day 32 through to Lord's Day 52. It discusses:
- Conversion (Lord's Days 32-33)
- The Ten Commandments (Lord's Days 34 - 44)
- The Lord's prayer (Lord's Days 45 - 52)
Lord's Day 1 
The first Lord's Day should be read as a summary of the catechism as a whole. As such, it illustrates the character of this work, which is devotional as well as dogmatic or doctrinal. The first Question and Answer reads:
"What is your only comfort in life and death?"
The answer is:
"That I am not my own, but belong--body and soul, in life and in death, to my faithful Savior Jesus Christ. He has fully paid for all my sins with his precious blood, and has set me free from the tyranny of the devil. He also watches over me in such a way that not a hair can fall from my head without the will of my Father in heaven; in fact, all things must work together for my salvation. Because I belong to him, Christ, by His Holy Spirit, assures me of eternal life and makes me whole-heartedly willing and ready from now on to live for him."
Use in various denominations and traditions 
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The Heidelberg Catechism is one of the three Reformed confessions that form the doctrinal basis of the original Reformed church in The Netherlands, and is recognized as such also by the Dutch Reformed churches that originated from that church during and since the 19th century.
Several Protestant denominations in North America presently honor the Catechism officially: the Presbyterian Church in America, the Christian Reformed Church, the United Reformed Churches, the Presbyterian Church (USA), the Reformed Church in America, the Communion of Reformed Evangelical Churches, the United Church of Christ (a successor to the German Reformed churches), the Reformed Church in the United States (also of German Reformed heritage), the Free Reformed Churches of North America, the Heritage Reformed Congregations, the Canadian and American Reformed Churches, and several other Reformed churches of Dutch origin around the world.
A revision of the catechism was prepared by the Baptist minister, Hercules Collins. Published in 1680, under the title 'An Orthodox Catechism', it was identical in content to the Heidelberg catechism, with exception to questions regarding baptism, where adult immersion was defended against infant baptism and the other modes of affusion and aspersion.
- Emil Sehling, ed., Die evangelischen Kirchenordnungen des XVI. Jahrhunderts, Band 14, Kurpfalz (Tübingen: J. C. B. Mohr (Paul Siebeck), 1969), 343: “Und demnach mit rhat und zuthun unserer gantzen theologischen facultet allhie, auch allen superintendenten und fürnemsten kirchendienern einen summarischen underricht oder catechismum unserer christlichen religion auß dem wort Gottes beides, in deutscher und lateinisher sprach, verfassen und stellen lassen, damit fürbaß nicht allein die jugendt in kirchen und schulen in solcher christlicher lehre gottseliglichen underwiesen und darzu einhelliglichen angehalten, sonder auch die prediger und schulmeister selbs ein gewisse und bestendige form und maß haben mögen, wie sie sich in underweisung der jugendt verhalten sollen und nicht ires gefallens tegliche enderungen fürnemen oder widerwertige lehre einfüren.”
- Lyle Bierma, “The Purpose and Authorship of the Heidelberg Catechism,” in An Introduction to the Heidelberg Catechism: Sources, History, and Theology (Grand Rapids: Baker, 2005), 67.
- J.F. Gerhard Goeters, “Zur Geschichte des Katechismus,” in Heidelberger Katechismus: Revidierte Ausgabe 1997, 3rd ed. (Neukirchen-Vluyn: Neukirchener Verlag, 2006), 89.
- Mark A. Noll, Confessions and Catechisms of the Reformation,(Vancouver, B. C.: Regent College Publishing, 1991), 134. ISBN 1-57383-099-2
- FRC: Heidelberg Catechism - Historical Background
- "Heidelberg Catechism". Encyclopedia Americana. 1920.