The Bible does warn against being lazy. In Proverbs 6:6-11, we are told to,
"Go to the ant, thou sluggard; consider her ways, and be wise: Which having no guide, overseer, or ruler, Provideth her meat in the summer, and gathereth her food in the harvest. How long wilt thou sleep, O sluggard? when wilt thou arise out of thy sleep? Yet a little sleep, a little slumber, a little folding of the hands to sleep: So shall thy poverty come as one that travelleth, and thy want as an armed man."
We are here cautioned against the negative consequences of laziness, but I can hear several voices at once, saying, just because it's bad practice doesn't necessarily mean we're commanded not to do it. Thus, our laziness doesn't necessarily count as a sin.
My nature is one to make similar arguments at every turn myself (immediately discounting them of course), so I was still a little skeptical of whether laziness itself should be considered a sin - an unwise practice most definitely, but was it a sin?
As I kept reflecting upon this, God showed me that I was looking at it all wrong. At its core, laziness shows a failure to commit to a cause. It highlights an attitude of selfishness, saying that my effort and time is more important than whatever task we are supposed to be doing.
When we work to serve the Lord, we best not be doing it lazily. We best not be wanting to put our time and effort above the Lord's will. We best not be trying to get away with the bare minimum.
Yet in Colossians 3:22-24, we hear,
"Servants, obey in all things your masters according to the flesh; not with eyeservice, as menpleasers; but in singleness of heart, fearing God; And whatsoever ye do, do it heartily, as to the Lord, and not unto men; Knowing that of the Lord ye shall receive the reward of the inheritance: for ye serve the Lord Christ."
Well, that's a commandment to do everything like we would do for the Lord, all heartily. I guess laziness is a sin after all.