Why is it called a mystery? Well it is because it is hidden. ‘In whom,’ in Christ, ‘are hid all the treasures of wisdom and knowledge.
Now it is not so hidden, even a worldling who has never come to Christ can understand, in its outline, the plan of salvation. ‘Oh I know what you teach: that God in Christ came from heaven and suffered and died for sinners.’ They can see the theory, they can see it in outline, but they can never really get it and grasp it in all its depth and wonder and feel it and its scope; it is hidden from them. Not so hidden that they are not responsible for rejecting Christ, but the riches of it and the glory of it come home only to believers. But you must have that, you must deeply appreciate it. The plan of salvation and the cost to Christ and what he did. It is the great antidote to error to see it, possess it, to fall at the feet of the Lord in wonder love and praise.