making known to us the secret of his will, according to his good pleasure which he set before in him
making known to us the secret of his will - False teachers were saying that God's will was that Israel was to inherit the earth and that there was secret knowledge that some could not know. Paul turns this around and says that the secret was that God's will was to save Jew and Gentile through Christ and that He made it known to all.
- Ephesians 1:9
according to his good pleasure - In contrast to the wrath that they thought would come upon the Gentiles during the Day of Destruction, Paul says that it was His good pleasure to save the Gentiles through Christ.
which he set before in him - The word used for "set before" in the Greek is protithēmi and mirrors the word proelpizō (See Eph. 1:12). The word means "to determine" or to set your mind to do something ahead of time (before). In this case the context suggests, that God set it before the foundation of the world (See Eph. 1:4). "In him" refers to God. Nobody but God had determined the plan of salvation through Christ.