The word “jealousy” has a bad connotation: we usually picture a jealous person as one who is short-tempered, unreasonable, unnaturally possessive. But God is sinless and holy, so what does He mean when He proclaims Himself to be a jealous God?
Even among humans there is a right kind of jealousy which is a “zeal to protect a love relationship” and sets safeguards to do so. But “God’s jealousy is not a compound of frustration, envy, and spite, as human jealousy so often is, but appears instead as a (literally) praiseworthy zeal to preserve something supremely precious” (p. 170).
Even humans expect loyalty in certain relationships. God certainly does as well, and as absolutely perfect and holy, He “will vindicate his claim by stern action against them if they betray his love by unfaithfulness” (p. 171).
Christians’ proper response, Packer says, should be zealousness for Him and His name. This doesn’t mean we start wars with people who don’t believe as we do. But it does mean we should have “a burning desire to please God, to do His will, and to advance His glory in the world in every possible way” (p. 173).
Chapter 17 ended a section of chapters called “Behold Your God!” which talked about His attributes; Chapter 18 begins the final section of the book called “If God Be For Us.”
The title of this chapter is “The Heart of the Gospel,” and Packer asserts that heart is propitiation. Like many people, I think the average preacher and writer needs to use words accessible to the common man and not lapse into verbiage only a theology student would grasp, but propitiation is a word that we need to understand. Packer defines it as “averting God’s anger by an offering” (p. 180), yet it is different from offerings for the same purpose in pagan religions due to many factors. One is that God initiated the offering of His Son. There is nothing we could offer that would take away our sin.
Some dislike the word and its concept because they don’t like to think of God as angry over sin. But as we have seen in the chapter on God’s wrath, God has a righteous and just anger over sin.
Propitiation was accomplished by the death of Jesus Christ on the cross, the OT sacrifices being a picture of that which was to come:
Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree (Galatians 3:13).
For the love of Christ constraineth us; because we thus judge, that if one died for all, then were all dead: And that he died for all, that they which live should not henceforth live unto themselves, but unto him which died for them, and rose again… To wit, that God was in Christ, reconciling the world unto himself, not imputing their trespasses unto them; and hath committed unto us the word of reconciliation. Now then we are ambassadors for Christ, as though God did beseech you by us: we pray you in Christ’s stead, be ye reconciled to God. For he hath made him to be sin for us, who knew no sin; that we might be made the righteousness of God in him. 2 Corinthians 5:14-15, 19-21).
Being justified freely by his grace through the redemption that is in Christ Jesus: Whom God hath set forth to be a propitiation through faith in his blood, to declare his righteousness for the remission of sins that are past, through the forbearance of God; To declare, I say, at this time his righteousness: that he might be just, and the justifier of him which believeth in Jesus (Romans 3:24-26).
And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world (1 John 2:2).
In this was manifested the love of God toward us, because that God sent his only begotten Son into the world, that we might live through him. Herein is love, not that we loved God, but that he loved us, and sent his Son to be the propitiation for our sins (1 John 4:9-10).
This was a very long chapter and I am only feebly hitting a few of the highlights here, but Packer gives a very thorough looking into from various angles.