Prerequisite
Part of speech: adjective
Definition: required as a prior condition of something else; necessary or indispensable
Part of speech: noun
Definition: Something that must be gained in order to gain something else In education, a course or topic that must be completed before another course or topic can be started. May be colloquially referred to as a prereq.
Example sentence: Testimony is the beginning of and a prerequisite to continuing conversion. Testimony is a point of departure; it is not an ultimate destination. Strong testimony is the foundation upon which conversion is established. Testimony alone is not and will not be enough to protect us in the latter-day storm of darkness and evil in which we are living.
Obligatory
Part of speech: adjective
Definition: Imposing obligation, morally or legally; binding: an obligatory promise.Requiring a matter or obligation.