From Euan Cameron, The European Reformation (1991):
Marriage was traditionally seen as a sacrament on the grounds that it was an analogy of the relationship between Christ and the Church. The reformers denied that this was sufficient reason to turn it into a ‘sacrament’ as such. Calvin mischievously remarked that on those grounds burglary would be a sacrament, since ‘the day of the Lord comes like a thief in the night.’