In Matthew 13 Jesus speaks a number of parables concerning the kingdom. The word "kingdom" has a primary definition and a secondary definition. We do well to understand both.
Primary Definition. Kingdom (basileia) is "primarily an abstract noun, denoting 'sovereignty, royal power, dominion'" (Vine's). The literal, primary definition of "kingdom" is "rule" (Strong's).
Secondary Definition. The secondary definition of kingdom (basileia) denotes the "territory or people over whom a king rules" (Vine's). The literal, secondary defintion of "kingdom" is "realm" (Strong's).
To enter the realm one must accept the rule.
Jason Cicero
