It is clear from Scripture that we are conceived condemned. That is, people are conceived covenantally guilty before God, even having done nothing, because of Adam's sin. Adam is the head of all the human race, as Paul makes quite clear in Romans 5 - and the headship is a covenantal headship as is made plain in that passage. We aren't talking mere "organic biology", but covenantal headship. We ALL, the Word says, sinned in Adam. Period. We are held accountable for his sin, and it is every bit as much our OWN sin, as it would have been had we been in his place.
Now if Adam, pre-fall, was in relationship with God under the terms of the same covenant that we are... then what does his breaking of that covenant do? Paul makes clear that we are guilty before God of Adam's sin. If his sin was a failure of faith, then we cannot be justified through faith. We cannot somehow supercede Adam's failure with our own success and sit just before God. We are conceived UNJUST - and therefore in need of a DIFFERENT covenantal arrangement.
The headship of Adam in covenant relationship with God implies, for his posterity, that in whatever the arrangement was, since he failed and broke that covenant, we, too, have broken that covenant. If there is to be a new covenant relationship such that people can be saved and brought into eternal relationship with God, then that new covenant CANNOT have BOTH the same promise and same conditions as the previous covenant. (else how is it new?) That covenant between God and Adam in the garden is done. Gone. Broken for all men who proceeded naturally from Adam, as he, their head failed to uphold its terms.
Monocovenantalism simply FAILS on the face of it. There is no way that Adam faced the same covenantal obligations in the garden, prior to his fall, that believers do today, post-fall. To argue this is to completely misread Genesis 3 and Romans 5 (among other places). To argue this is to destroy the covenant headship of Adam, and to twist the covenant headship of Christ into something unrecognizable.
What's coming next in The Marrow of Modern Divinity is the discussion of the promise of God. In that promise was revealed several important things: Thomas Boston, in the notes presented on page 68 of the version one can purchase here, writes:
"In this promise was revealed, 1. Man's restoration unto the favour of God, and his salvation; not to be effected by man himself, and his own works, but by another. For our first parents, standing condemned for breaking of the covenant of works, are not sent back to it, to essay the mending of the matter, which they had marred before; but a new covenant is purposed,—a Saviour promised as their only hope. 2. That this Saviour was to be incarnate, to become man, "the seed of the women." 3. That he behoved to suffer; his heel, namely his humanity, to be bruised to death. 4. That by his death he should make a full conquest over the devil, and destroy his works, who had now overcome and destroyed mankind; and so recover the captives out of his hand: "he shall bruise thy head, viz: while thou bruisest his heel." This encounter was on the cross: there Christ treading on the serpent, it bruised his heel, but he bruised its head. 5. That he should not be held by death, but Satan's power should be broken irrecoverably: the Saviour being only bruised in the heel, but the serpent in the head. 6. That the saving interest in him, and his salvation, is by faith alone, believing the promise with particular application to one's self, and so receiving him, forasmuch as these things are revealed by way of a simple promise." (p. 68, The Marrow of Modern Divinity)This promise is simple news - the gospel - good news of God's redeeming work, which He accomplished in the sending of His Son for His people. This ancient gospel, the covenant of grace first announced in Genesis 3:15, and subsequently revealed progressively through the history of God's covenant people, by announcements from prophets and priests (and kings), is the next subject of the Marrow.