Simplify the following text for a Grade 10 native English-speaking person.
Original: They argue that the custom of meeting for worship on Sunday originated in paganism , specifically Sol Invictus and Mithraism ( in which sun-god worship took place on Sunday ) and constitutes an explicit rejection of the commandment to keep the seventh day holy .
Simplified: They argue that the custom of meeting for worship on Sunday originated in paganism , specifically Sol Invictus and Mithraism ( in which sun-god worship took place on Sunday ) . This is therefore an explicit rejection of the commandment to keep the seventh day holy .