I have some historical reasons, some biblical reasons, and some theological reasons. But let me first start with what I mean by “it is a sin”: First, I am not a Sabbatarian, meaning I do not believe the Sabbath Law from the Old Covenant carries over into the New Covenant. If I were, then I would simply say the Fourth Commandment is where God commands us to gather on one day out of the week for worship, and that we can infer from the early church and from the resurrection that the Sabbath has moved from Saturday to Sunday. But as I said, I am not a Sabbatarian.
I am, however, what I would consider Covenantal, in that I do believe God put an eternal Law on every human heart, from Adam until now (in other words, God made a Law-Covenant with Adam and all his progeny at creation). Because of this, every human being knows that one of the laws God wrote on our hearts is that we must worship God (Romans 1.18-25). Anyone who does not gather for worship knows they are breaking God’s Law before you even tell them explicitly. I think that issue right there should persuade the majority of people who break this command. Most people who do not gather on Sundays are not gathering because they simply do not want to worship God.
But how do I gather that it is a sin to not worship specifically on the Lord’s Day (Sunday, the first day of the week), since I am not a Sabbatarian? All I can say is that I infer this from Scripture. We can infer/understand/conclude from Scripture that the Lord wants His people to gather on the Lord’s Day to worship. If we know the Lord wants something, and we do not do it, then, yes, it is sin. If we know the good we ought to do, and do not do it, then, yes, it is sin.
We do not need an explicit command from Scripture to tell us what to do on every important issue, do we? The bible does not explicitly command fasting, nor how many times a year we must observe the Lord’s Supper, nor does it explicitly prohibit french kissing someone who is not your spouse. Yet, I think we can infer what the Lord wants from us on those issues. On top of that, I will argue that “infer” is a weak word for the Lord’s Day gathering “command.” There is a lot more than meets the eye in Scripture on this issue.
I am probably setting myself up for failure by promising so much, but by God’s grace, if I am even able to make you appreciate the weight of this issue a little more, I will be happy. My goal is really to get you to say, “why on earth would I not gather to worship on the Lord’s Day?”