I ...; one ...
Archaic/literary expression where が marks the subject; equivalent to modern 自分が. Rare outside classical or highly stylized writing.
Believe in the path you yourself walk.
Archaic/literary possessive or subject marker; が is part of the fixed form, not a modern particle.
From classical Japanese 己 (ono, 'self') + possessive/subject particle が. The form is fossilized and not productive in modern Japanese.