9 Having made known unto us the mystery of his will, according to his good pleasure which he hath purposed in himself:
God’s good pleasure is why we are here.
Revelation 4:11 Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created.
It is God’s will and desire that all men should all be saved.
1Timothy 2:4 Who will have all men to be saved, and to come unto the knowledge of the truth.
Although, clearly, not all will so all in 1 Timothy 2:4 is all without distinction, not all without exception, and about this point regarding God’s will Calvinists are wrong. No one was predestinated to an eternity of agony by anything other than their own will.
2Thessalonians 1:8 In flaming fire taking vengeance on them that know not God, and that obey not the gospel of our Lord Jesus Christ:
That He was willing to save all men and not just the Jews, His chosen people, was a mystery that few understood. The church, the body of Christ on earth, was a mystery.
Romans 16:25 ¶ Now to him that is of power to stablish you according to my gospel, and the preaching of Jesus Christ, according to the revelation of the mystery, which was kept secret since the world began, 26 But now is made manifest, and by the scriptures of the prophets, according to the commandment of the everlasting God, made known to all nations for the obedience of faith: 27 To God only wise, be glory through Jesus Christ for ever. Amen.
That God would make the sacrifice for sins that would make salvation available to all mankind was a secret.
Deuteronomy 29:29 The secret things belong unto the LORD our God: but those things which are revealed belong unto us and to our children for ever, that we may do all the words of this law.
Now we know some of what God will reveal to us but when we stand before Christ we’ll know it all.
1Corinthians 13:12 For now we see through a glass, darkly; but then face to face: now I know in part; but then shall I know even as also I am known.
That being said, the “mystery of his will” in this context is revealed in the next verse.
10 That in the dispensation of the fulness of times he might gather together in one all things in Christ, both which are in heaven, and which are on earth; even in him:
This is the mystery of his will, that in eternity, the dispensation of the fullness, the completion of, all time He gathers together in one all things in Christ.
This all is all without distinction, not all without exception, as Satan, the Beast, and the unsaved dead will not be gathered together to Him.
1 Corinthians 15:24 Then cometh the end, when he shall have delivered up the kingdom to God, even the Father; when he shall have put down all rule and all authority and power. 25 For he must reign, till he hath put all enemies under his feet. 26 The last enemy that shall be destroyed is death. 27 For he hath put all things under his feet. But when he saith all things are put under him, it is manifest that he is excepted, which did put all things under him. 28 And when all things shall be subdued unto him, then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
He will bring all things that are “in Christ”, dead and living, to Him.
11 In whom also we have obtained an inheritance, being predestinated according to the purpose of him who worketh all things after the counsel of his own will:
An inheritance is something physical as in;
Genesis 15:7 And he said unto him, I am the LORD that brought thee out of Ur of the Chaldees, to give thee this land to inherit it.
You can inherit a spiritual thing as in;
Luke 18:18 And a certain ruler asked him, saying, Good Master, what shall I do to inherit eternal life?
But, is eternal life a purely spiritual thing or is it also physical?
1 Corinthians 15:52 In a moment, in the twinkling of an eye, at the last trump: for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.53 For this corruptible must put on incorruption, and this mortal must put on immortality.
Romans 8:23 And not only they, but ourselves also, which have the firstfruits of the Spirit, even we ourselves groan within ourselves, waiting for the adoption, to wit, the redemption of our body.
1 Peter 1:3 ¶ Blessed be the God and Father of our Lord Jesus Christ, which according to his abundant mercy hath begotten us again unto a lively hope by the resurrection of Jesus Christ from the dead, 4 To an inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you,
Now, this is an inheritance in eternity to which you have been elected by your trusting in Christ.
In this verse your inheritance is something to which you are predestinated because of your faith in Christ. Predestination always refers to what happens to you after you are saved. Your destiny is not fixed until you are born again. You have a choice to make and then after your salvation certain choices are made for you but you still have a great deal of free will, so you can’t get out of responsibility for your actions.
When you go to heaven you can go first class, tourist, or in the baggage compartment, but you are going to heaven if you are saved. Period.
12 That we should be to the praise of his glory, who first trusted in Christ.
The very fact that this first group of Jewish and Gentile Christians trusted in Christ is a praise to His eternal glory. The Holy Spirit drew them out of the corrupt world system of Gentile paganism and Jewish pharisaical self-righteousness against all odds. There was no political power and no advantage to be gained, only persecution, suffering, and death outside of the joy of following Christ and the promise of life eternal. We would do well to look into the early church for a comparison with what passes for Christianity today.