...he made known to us the mystery of his will according to his good pleasure, which he purposed in Christ…
The word "mystery" is translated from the Greek word MYSTRIUM, which according to Vine's: "...denotes, not the mysterious (as with the English word), but that which, being outside the range of unassisted natural apprehension, can be made known only by divine revelation, and is made known in a manner and at a time appointed by God, and to those only who are illumined by His Spirit."
The Lord has provided an open invitation for all people to come to Him and experience a spiritual rebirth -- which is a part of the process in understanding the "mystery of His will." Unfortunately, anyone who rejects Jesus is also refusing to recognize that He is the source of all wisdom and knowledge. In other words, the mysteries of God are reserved for only those who faithfully receive Jesus and then draw on His wisdom and understanding through the Holy Spirit. Knowing Christ is the key to unlocking the mystery of God's will. (Colossians 2:9-10: "For in Christ all the fullness of the Deity lives in bodily form, and in Christ you have been brought to fullness.)
I think one of the great mistakes that Christians make is trying to understand God and His will by using human intellect alone. Do you think we have a chance to comprehend God's will without Jesus? I don't. Will you seek Christ and His wisdom? How could this insight adjust the way you approach the Lord?