The first is about baptism (yes, again). The Great Commission reads:
And Jesus came and said to them, “All authority in heaven and on earth has been given to me. Go therefore and make disciples of all nations, baptizing them in the name of the Father and of the Son and of the Holy Spirit, teaching them to observe all that I have commanded you. And behold, I am with you always, to the end of the age.”Matthew 28:18-20 (ESV)
This commandment is threefold: go, teach, baptize. Many churches are happy with two out of three. They're happy to evangelize and disciple, but they're not so keen on baptism. But Jesus clearly instructs us to do all three. Putting aside questions of why we baptize, or how, the Church is told to do it. Wouldn't that make not doing it a sin of ommission?
The second is about attendance. We often quote Hebrews 10:25--"not forsaking the assembling of ourselves together"--when teaching about the need to attendance services regularly, but the word "church" itself instructs us to do so. Well, not the English word "church." That's derived from a word meaning a lord's manor (any lord's). The word translated church is actually "ekklesia" in the original Greek. Church groups were called "ekklesia." The word was used in Greek civic politics to describe those citizens called together to do the city's business. A town meeting. A "church" is a called assembly.
Let's compare it to baptism. We baptize by immersion because the word itself means "to immerse." Likewise, we gather together to worship because a church, or an ekklesia, means to assemble. You can't have an assembly all by yourself.