James Ussher on the covenant of works

"Declare now out of that which hath been said, what the covenant of works is? 

It is a conditional covenant between God and man, whereby on the one side God commandeth the perfection of godliness and righteousness, and promiseth that he will be our God, if we keep all his commandments; and on the other side, man bindeth himself to perform entire and perfect obedience to God's law, by that strength wherewith God hath indued him by the nature of his first creation."(James Ussher, A Body of Divinity)


"What employment had man in this estate? 

A two fold employment: The first outward, to till and dress the garden, Gen. 2. 15. The other spiritual, to worship and serve God his Creator, and to procure his own everlasting blessedness; whereto he was fitted with freedom of will and ability for perfect obedience unto God, according to the tenor of the covenant of works." (James Ussher, A Body of Divinity; underlining mine--CD)

What becomes of the work of Jesus Christ on Ussher's vain and speculative notion that man (Adam) procures "his own everlasting blessedness"?  Does not this supposition posit the possible erasure of the work of Jesus Christ from history?  Ussher implies that God works against Himself by fitting man with a (presumed) "freedom of will and ability" to thwart His "eternal purpose which He accomplished in Christ Jesus our Lord" (Ephesians 3:11).