A hadith and commentary.
The Prophet, peace and blessings of Allah be upon him, said:
"Whoever dies without associating anything with Allah,
shall enter Paradise."
"And whoever dies while associating anything with Him,
shall enter the Fire."
This was narrated by Muslim, and al-Bukhari narrated a portion of it.
Commentary:
The Oneness of Allah (tawhid)
is the foremost of the religious commandments and the most deserving of our attention.
As for associating partners with Him,
it is the greatest of major sins, the most heinous of evils, and the most detestable of all wrongs.
Therefore, the most essential of all duties
is that we learn tawhid so that we may draw closer to Allah through it,
and that we understand shirk so that we may avoid it and be wary of it.