A phrase used by Protestant theologians to distinguish between what they consider as essential articles of Christian faith and those which they consider non-essential.
Dissident churches might agree on the essential and thus form one body though disagreeing on the non-essential.
The difficulty is of course to get any two Protestant churches to determine what is and what is not essential.
It was in controversy on this point with the Calvinist Jurieu that Bossuet wrote his classic: "."
The Catholic position is that every revealed truth is fundamental or essential and that, no matter how less in importance it may seem than other truths, to reject it is to reject the word of God.
New Catholic Dictionary