That it shall be unlawful for any person, firm or corporation to keep open their establishment or place of business on the first day of the week, commonly called Sunday, for any of the purposes mentioned in section 1 of this act: Provided, however, That nothing in this act shall apply to any person who conscientiously believes that the seventh day of the week, from sunset Friday to sunset Saturday, should be observed as the Sabbath and who actually refrains from conducting or engaging in the business of buying, selling or offering for sale motor vehicles, or performing other secular business on that date.