The purpose of the "Act of Supremacy" is to establish the English monarch as the official head of the Church of England, supplanting the power of the Catholic pope in Rome.
Henry had sought in vain for papal approval for his divorce from Katherine of Aragon, and when it became clear that approval would not be forthcoming, Henry took matters into his own hands.
At the same time, the Church of England was poorly administered and had long since lost the right to act as an independent body.And
Henry had his eye on the wealth of the church, particularly the property of the monasteries.
The Act of Supremacy, 1534, effectively made it treasonable to support the authority of the Pope over the Church of England. Therefore, support for Catholicism became not simply a statement of personal religious conviction, but a repudiation of the authority of the monarch, and as such, an act of treason punishable by death.
(abridged from http://www.britainexpress.com/History/tudor/act-of-supremacy.htm)
Henry VIII 's Act of Supremacy (1534) - original text
Albeit the king's Majesty justly and rightfully is and ought to be the supreme head of the Church of England, and so is recognized by the clergy of this realm in their convocations, yet nevertheless, for corroboration and confirmation thereof, and for increase of virtue in Christ's religion within this realm of England, and to repress and extirpate all errors, heresies, and other enormities and abuses heretofore used in the same, be it enacted, by authority of this present Parliament, that the king, our sovereign lord, his heirs and successors, kings of this realm, shall be taken, accepted, and reputed the only supreme head in earth of the Church of England, called Anglicans Ecclesia; and shall have and enjoy, annexed and united to the imperial crown of this realm, as well the title and style thereof, as all honors, dignities, preeminences, jurisdictions, privileges, authorities, immunities, profits, and commodities to the said dignity of the supreme head of the same Church belonging and appertaining; and that our said sovereign lord, his heirs and successors, kings of this realm, shall have full power and authority from time to time to visit, repress, redress, record, order, correct, restrain, and amend all such errors, heresies, abuses, offenses, contempts and enormities, whatsoever they be, which by any manner of spiritual authority or jurisdiction ought or may lawfully be reformed, repressed, ordered, redressed, corrected, restrained, or amended, most to the pleasure of Almighty God, the increase of virtue in Christ's religion, and for the conservation of the peace, unity, and tranquility of this realm; any usage, foreign land, foreign authority, prescription, or any other thing or things to the contrary hereof notwithstanding.
taken from http://www.britainexpress.com/History/tudor/supremacy-henry-text.htm
1559 Act of Supremacy
Henry VIII's Act of Supremacy was repealed (1554) in the reign of his staunchly Catholic daughter, Mary I. Unsurprisingly, it was reinstated by Mary's Protestant sister, Elizabeth I, when she ascended the throne. Elizabeth declared herself Supreme Governor of the Church of England, and instituted an Oath of Supremacy, requiring anyone taking public or church office to swear allegiance to the monarch as head of the Church and state. Anyone refusing to take the Oath could be charged with treason.
There were three levels of penalties for refusal to take the Oath of Supremacy. A first refusal to resulted in loss of all movable goods. A second offence could mean life in prison and a loss of all real estate Possessions. A third offence would result in a charge of High Treason and death. A few years later the Oath was extended to include M.P.s and anyone taking a university degree.
(abridged from http://www.britainexpress.com/History/tudor/act-of-supremacy.htm)
Elizabeth I's Act of Supremacy (1559) - original text
An acte restoring to the crown the ancient jurisdiction over the state ecclesiastical and spiritual and abolishing all foreign power repugnant to the same. Most humbly beseech your most excellent majesty your faithful and obedient subjects, the lords spiritual and temporal and the commons in this your present parliament assembled, that, where in time of the reign of your most dear father of worthy memory, King Henry VIII, divers good laws and statutes were made and established, as well for the utter extinguishment and putting away of all usurped and foreign powers and authorities out of this your realm and other your highness's dominions and countries, as also for the restoring and uniting to the imperial crown of this realm the ancient jurisdictions, authorities, superiorities, and pre-eniinences to the same of right belonging and appertaining; by reason whereof we, your most humble and obedient subjects, from the five-and-twentieth year of the reign of your said dear father, were continually kept in good order, and were disburdened of divers great and intolerable charges and exactions before that time unlawfully taken and exacted by such foreign power and authority as before that was usurped, until such time as all the said good laws . . . in the first and second years of the reigns of the late King Philip and Queen Mary . . . were . . repealed . . . ;' by reason of which act of repeal your said humble subjects were eftsoons brought under an usurped foreign power and authority, and vet do remain in that bondage, to the intolerable charges of your loving subjects, if some redress by the authority of this your high court of parliament with the assent of your highness be not had and provided: may it therefore please your highness, for the repressing of the said usurped foreign power and the restoring of the rights jurisdictions, and pre-eminences appertaining to the imperial crown of this your realm, that it may be enacted by the authority of this present parliament that the said act . . . and all and every branch, clauses, and articles therein contained, other than such branches, clauses, and sentences as hereafter shall be excepted, may from the last day of this session of parliament, by authority of this present parliament, be repealed, and shall from thenceforth be utterly void and of none effect . . .
And to the intent that all usurped and foreign power and authority,spiritual and temporal, may forever be clearly extinguished and never to be used nor obeyed within this realm or any other your majesty's countries, may it please your highness that it may be further enacted by the authority aforesaid that no foreign prince,person, prelate, state, or potentate, spiritual or temporal, shall at any time after the last day of this session of parliament use, enjoy, or exercise any manner of power, jurisdiction, superiority, authority, pre-eminence, or privilege, spiritual or ecclesiastical, within this realm or within any other your majesty's dominions or countries that now be or hereafter shall be, but from thenceforth the same shall be clearly abolished out of this realm and all other your highness's dominions forever, any statute, ordinance, custom, constitutions, or any other matter or cause whatsoever to the contrary in any wise notwithstanding . . . ; and that your highness, your heirs, and successors, kings or queens of this realm, shall have full power and authority . . . to exercise . . . all manner of jurisdictions, privileges, and preeminences in any wise touching or concerning any spiritual or ecclesiastical jurisdiction within these your realms. . . .
And for the better observation and maintenance of this act, may it please your highness that it may be further enacted by the authority aforesaid that all and every archbishop, bishop, and all and every other ecclesiastical person and other ecclesiastical officer and minister, of what estate, dignity, pre-eminence, or degree soever he or they be or shall be, and all and every temporal judge, justicer, mayor, and other lay or temporal officer and minister, and every other person having your highness's fee or wages within this realm or any your highness's dominions shall make, take, and receive a corporal oath upon the Evangelist, before such person or persons as shall please your highness, your heirs or successors, under the great seal of England to assign and name to accept and take the same, according to the tenor and effect hereafter following, that is to say-
"I, A. B., do utterly testify and declare in my conscience that the queen's highness is the only supreme governor of this realm and of all other her highness's dominions and countries, as well in all spiritual or ecclesiastical things or causes as temporal, and that no foreign prince, person, prelate, state, or potentate hath or ought to have any jurisdiction, power, superiority, pre-eminence, or authority, ecclesiastical or spiritual, within this realm; and therefore I do utterly renounce and forsake all foreign jurisdictions, powers, superiorities, and authorities, and do promise that from henceforth I shall bear faith and true allegiance to the queen's highness, her heirs, and lawful successors, and to my power shall assist and defend all jurisdictions, pre-eminences, privileges, and authorities granted or belonging to the queen's highness, her heirs, and successors, or united or annexed to the imperial crown of this realm: so help me God and by the contents of this Book." . . .
And for the more sure observation of this act and the utter extinguishment of all foreign and usurped power and authority, may it please your highness that it may be further enacted by the authority aforesaid that, if any person or persons dwelling or inhabiting within this your realm or in any other your highness's realms or dominions . . . , shall by writing, printing, teaching, preaching, express words, deeds, or act, advisedly, maliciously, and directly affirm, hold, stand with, set forth, maintain, or defend the authority, preeminence, power, or jurisdiction, spiritual or ecclesiastical, of any foreign prince, prelate, person, state, or potentate whatsoever, heretofore claimed, used, or usurped within this realm or any dominion or country being within or under the power, dominion, or obeisance of your highness, or shall advisedly, maliciously, or directly put in ure or execute anything for the extolling, advancement, setting forth, maintenance, or defence of any such pretended or usurped jurisdiction, power, pre-eminence, or authority, or any part thereof, that then every such person and persons so doing and offending, their abettors, aiders, procurers, and counsellors, being thereof lawfully convicted and attainted according to the due order and course of the common laws of this realm [shall suffer specified penalties, culminating in punishment for high treason on the third offence] . . . .
Provided always, and be it enacted by the authority aforesaid, that such person or persons to whom your highness, your heirs, or successors, shall hereafter by letters patents under the great seal of England give authority to have or execute any jurisdiction, power, or authority spiritual, or to visit, reform, order, or correct any errors, heresies, schisms, abuses, or enormities by virtue of this act, shall not in any wise have authority or power to order, determine, or adjudge any matter or cause to be heresy but only such as heretofore have been determined, ordered, or adjudged to be heresy by the authority of the canonical Scriptures, or by the first four general councils or any of them, or any other general council wherein the same was declared heresy by the express and plain words of the said canonical Scriptures, or such as hereafter shall be ordered, judged, or determined to be heresy by the high court of parliament of this realm, with the assent of the clergy in their convocation - anything in this act contained to the contrary notwithstanding...
taken from http://www.britainexpress.com/History/tudor/supremacy-text.htm