The Talmud states, "Cover your head in
order that the fear of heaven may be
upon you. "Rabbi Hunah ben Joshua never
walked 4 cubits (2 meters) with his head
uncovered. He explained: 'Because the
Divine Presence is always over my head.
Jewish Law dictates that a man is
required to cover his head during
prayer. Originally, a head covering at
other times for Orthodox males was a
custom, but it has since taken on "the
force of law" because it is an act of
Kiddush Hashem. The 17th-century
authority David HaLevi Segal suggested
that the reason was to distinguish Jews
from their non-Jewish counterparts,
especially while at prayer.
According to the Shulchan Arukh, Jewish
men are strongly recommended to cover
their heads, and doing so, should not
walk more than four cubits bareheaded.
Covering one's head, such as by wearing
a kippah, is described as "honoring
God". The Mishnah Berurah modifies this
ruling, adding that the Achronim
established it as a requirement to wear
a head covering even when traversing
less than four cubits, and even when one
is standing still, indoors and outside.
Kitzur Shulchan Aruch cites a story from
the Talmud (Shabbat 156b) about Rav
Nachman bar Yitzchok who might have
become a thief had his mother not saved
him from this fate by insisting that he
cover his head, which instilled in him
the fear of God. In many communities,
boys are encouraged to wear a kippah
from a young age in order to ingrain the
habit.
More lenient opinions also exist, and
many great rabbis did not wear a head
covering. The GRA or Vilna Gaon says one
can make a berakhah without a kippah)
and other poskim, and wearing a kippah
is only a midos chassidus (exemplary
attribute). Recently, there seems to
have been an effort to suppress earlier
sources that practiced this leniency,
including erasing lenient responsa from
newly published books.
According to Rabbi Isaac Klein's Guide
to Jewish Religious Practice, a
Conservative Jew ought to cover his head
when in the synagogue, at prayer or
sacred study, when engaging in a ritual
act, and when eating. In the mid-19th
century, Reformers led by Rabbi Isaac
Wise stopped wearing kippot altogether.