When thinking about the 'vow' of Zinzendorf's Order of the Mustard Seed, some people raise the question 'what about Jesus' command to his followers that we should not swear oaths at all?' (Matt 5:33-37). It's a fair point, and one which deserves an answer.
Making 'vows' as a voluntary act of devotion to God is regarded favourably in both the Old and New testaments. The Nazirite vow (Num 6) was taken by Samson and on behalf of John the Baptist. Paul and other early disciples seem to have made vows which had similar chracteristics (at least we know the hair bit was involved, Acts 18:18, 21:22-24). Contrast these acts of consecration to the 'oaths' Jesus is referring to, which were used to shore up statements coming from a place of dubious personal integrity (and often came with neat legal get-out clauses, Matt 23:15-22).
We would understand the Mustard seed vow as falling into the first category; a voluntary and valuable act of consecration and devotion to God, which is a reflection and expression of the 'new covenant' commitment that is the heart of what it means to be a Christian believer. It is not an oath in the sense of 'swearing by' someone or something, to which Jesus was referring.
Interestingly, the 18th century Moravians who first lived by this 'vow' held very strong convictions against swearing oaths. This sometimes caused them problems with the authorities; for example, they refused to take the oath of allegiance required of those who wanted to settle in British overseas colonies. It took an Act of Parliament to sort that one out and allow them freedom to settle while repecting their moral scruples.
