The Third Commandment of the Reformed Decalogue was ‘Thou shalt not take the name of the LORD thy God in vain’. It would therefore be easy to assume that the primary purpose of the commandment was to forbid blasphemy – employing God’s name in the uttering of profane curses or the swearing of false or deceitful oaths. In fact, the third commandment was much more sweeping in its scope than this. The Pembroke College graduate and Suffolk minister Robert Allen explained that the scope of the commandment was nothing less than ‘to shew what ought to be the ordinarie course of the of the whole life and conversation of the true worshipper of God, both in word and deed’. Secondly, it was
To declare what is the chiefe end of life, and of all the thoughts, words, & works thereof; not only in the duties of God’s worship, both inward & outward, according to the first and second Commandment: but also in every other duty according to all the Commandments of the whole Law of God.
The third commandment was therefore pretty totalising: it did not just apply to oaths and curses, but directed the whole ordinary life of the believer outside of the context of religious worship.
The ultimate purpose of the Christian life, according to Allen and other commentators, was the honour and glory of God. Although the text of the commandment directed the believer not to take the name of the lord in vain, by God’s name was to be taken not only ‘his divine title, word, and works’, but also ‘God himself, and whatsoever he hath by the same his divine titles word, and works, revealed itself’. This meant honouring not just the name of God, but holding in reverence his entire work of creation; from the world itself to the all of the creatures that populated it.
The positive aspect of the third commandment was therefore not so much about enjoining a particular action (employing God’s name in a holy way), but rather dictated that all true believers ought ‘to live to the glory of his name’, a totalising philosophy of godly behaviour and attitude. Accordingly, the negative aspect of the commandment did much more than simply forbid the sin of blasphemy. To take the lord’s name in vain was to ‘lightly and unreverendly to speak of God, his title, word, or works … or else in the course of life and conversation, to walke so slowly, indiscreetly, and unconstantly … that God hath no glory, but rather is dishonoured’.
Specific sins therefore included ‘light, foolish, and jesting speeches’, which put off sober religious contemplation. Abuse of God’s name included blasphemy and perjury, but extended beyond them to dice play, fortune telling, charms, sorcery, conjuration and witchcraft. Instead, believers were obliged to hold God and his titles, words, works and ordinances in reverend and worthy estimation. The commandment also required that the faithful live in a calling or occupation ‘as may well agree to the holy profession of Gods name’. And finally, it meant that ‘if by any occasion we fall into sin, to the dishonour of the name of God, that we confess and bewaile it, and so earnestly returne to God and his Church, that our repentance may be as notable and famous as our fall’.
The Third Commandment also contained an explicit curse (and an implicit blessing) for those who disobeyed (or obeyed) it: ‘the lord will not hold him guiltless that taketh his name in vain’. As Allen explained, by this threat, ‘the Lord giveth to understand, that he will severely punish every such one as shall any manner of way dishonour and abuse the most reverend and glorious Majestie of his name’. Particular biblical curses were articulated for blasphemers in general, the wicked, the vain, the impatient, perjurers, loose livers, godly succumbers to temptation, the unconstant, hypocrites, negligent governors, false prophets, enchanters, inciters of ungodly practices, the wicked, and blasphemers against the holy ghost. Such sinners could expect a first class ticket ‘even to the horrible pit of hell, if they will not turne unto God by speedie repentance’.
More positively, those who managed to obey the commandment ‘shall shine as the brightnesse of the firmament, and they that turne manie to righteousnesse shall shine as the stars for ever and ever’. Not a bad deal, for living a good and honest life and honouring God’s name…
 Robert Allen, A treasurie of catechism, or Christian instruction (1600), STC2: 366, p. 80.