Could Handel ever have imagined that a composition of his, which debuted in Dublin in 1742, would thereafter and worldwide be one of the most significant choral pieces performed during the Christmas season? “It’s like the marriage of all that’s good and wonderful in the world coming together when we hear the ‘Messiah,’ ” said the National Philharmonic Chorale’s artistic director, Stan Engebretson, who will conduct 170 voices along with the National Philharmonic Orchestra in what has become a ubiquitous annual tradition.