“sacred secret.” The Greek word mustērion (#3466 μυστήριον) means “sacred secret.” It refers to a secret in the religious or sacred realm.
[For more information on the “Sacred Secret” and the Administration of Grace, see commentary on Eph. 3:9.]
“those who are outside.” The “outside” is not referring to outside the Twelve, because more people than the Twelve were around him and were taught the sacred secrets of the Kingdom (cp. Mark 4:10). Rather, the outside refers to those outside Christ’s group, many of whom were not invested in following Christ. Some of them were new, some of them were only interested in seeing miracles and such, and some of them were detractors who did not believe what he was teaching.
“everything is in parables.” Jesus spoke in parables to reveal the hearts of the people who were hearing him speak. Humble, godly people found out what the parables meant while pious, arrogant people did not bother to find out.
[For more on Christ speaking in parables, see commentary on Matt. 13:13.]