It’s not a requirement in the same sense as for a Catholic, but our Protestant brothers and sisters are also required to ‘honour the Sabbath day’, if not by church, at least by prayer.
I’ve been inside Evangelical Protestantism for eight years but I’ve never heard it suggested that missing church on Sunday is a sin. I would guess that most Evangelicals would think going to a Catholic church on a Sunday is a sin, though.
While missing a Sunday service would not necessarily be a sin for a Lutheran, the facts are 1) we are commanded to honor the sabboth, and 2) divine service is where we hear the Word and receive the Sacrament. So, consistently missing cuts one off from the means of grace, leading to greater alienation from God.
To intentiionally cut oneself off from God is obviously a “mortal” sin.
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