What is the theme of "Crossing the Bar?"

The theme of Tennyson's poem centers on the idea of
maturation and growth.  The journey that Tennyson articulates in the pom is one where
courage and maturation are needed in order to effectively cope with the reality that
lies outside the harbor, into the sea.  The "bar" that has to be crossed lies in this
voyage, and being able to understand that one might not be able to fully control it, but
rather be able to accept it as a part of being in the world is where the poem's strength
lies.  The unknown, and what lies outside the realm of the harbor, cannot be tamed nor
can it be avoided as all journeys enter "the boundless deep."  This cannot be averted
and is a part of the ship's purpose, and the journey of consciousness.  Tennyson
suggests that this is a phase that must be passed, with faith placed in the "Divine and
Unseen Who is always guiding us.”  In this, the poem's thematic application to death and
dying becomes something that resonates throughout, making the poem so
significant.