according to what pleased him

Possible meanings are 1) "because he wanted to make it known to us" or 2) "which was what he wanted."

which he demonstrated in Christ

"he demonstrated this purpose in Christ"

"by means of Christ"

A new sentence can be started here. AT: "He did this with a view to a plan" or "He did this, thinking about a plan"

"for when the time is right" or "for the time that he has appointed"