"Certainly, this Christ, his death and resurrection, including his ascension and Penetcost, as the culmination of redemptive history, are the heart-core of the gospel. They are, as Paul says, 'of first importance' (1 Cor 15:3). He and other New Testament writers make that abundantly clear. The controlling centrality of Christ's work is not at issue here.Gaffin stresses, along with Calvin, the centrality of Christ's death achieving for us Union with Christ. What is it to be a Christian? To be in union with Christ. Joined irrevocably to him - that is real new life! To live is Christ. And in this life, I will boast in the cross.
But to punctuate the gospel, particularly its proclamation, with a full stop after Christ's death, resurrection and ascenion, allowing for his future return, does not do the gospel full justice, as 'thepower of God for the salvation of everyone who believes' and as that gospel involves the revelation of the righteousness of God 'from faith to faith' (Rom. 1:16-17). In fact, as Calvin intimates, that sort of parsing of the gospel misses and integral component, something absoluetly essentail. Or, as subsequent Reformed theology affired with aphoristic pointedness: 'Without application, redemption is not redemption'." (p278)
He presses on in the theme with some great thoughts about the role of faith, the Spirit's power and the doctrine of justification.
"We observe 'that not all indiscriminately embrace that communion with Christ which is offered through the gospel'. Why is that? Not because of some differentiating factor on our side. The answer is not to be found by looking into ourselves or contemplating the mystery of human freedom and willing. Rather, consistent with his uniform teaching elsewhere about the total inability of the will due to sin, we must 'climb higher' and there consider 'the secret energy of the Spirit'. Faith then is Spirit-worked, sovereignly and effiaciously. The union Calvin has in view, then, is union forged by the SPirit's working faith in us, a faith that 'puts on' Christ (citing Gal 3:27), that embraces Christ as he is offered to faith in the gospel. Faith is the bond of that union seen from our sin. 'To sum up, the Holy Spirit is the bond by which Christ effectually unites us to himself.'" (p279)Gaffin pursues this further here: Biblical Theology and the Westminster Standards (at BeginningWithMoses.org. Next, to Calvin: The Way We Receive The Grace of Christ.