'Singleness' in the Bible

From Zebulun: 50,000 who could serve in the army, trained for battle with all kinds of weapons of war, with one purpose to help David.

And they, continuing daily with one accord in the temple, and breaking bread from house to house, did eat their meat with gladness and singleness of heart,

or he who encourages, in the act of encouragement; he who gives, with generosity; he who leads, with diligence; he who shows mercy [in caring for others], with cheerfulness.

Servants, be obedient to them that are your masters according to the flesh, with fear and trembling, in singleness of your heart, as unto Christ;

Servants, obey in all things your masters according to the flesh; not with eyeservice, as menpleasers; but in singleness of heart, fearing God: