While both the gross synergists (Melanchthon: “The assenting will of man is an efficient cause of conversion”) and the subtle synergists (Latermann, modern evangelical Protestantism in general : “Man’s will is able to decide for salvation through new powers bestowed by God”) profess to teach salvation by grace, they in reality disavow the Scriptural doctrine of salvation by grace and inculcate salvation by work-righteousness. Both the gross and the subtle synergists, in the final analysis, ascribe man’s salvation in part to his good conduct, his decision for Christ, his self-determination, his omission of malicious resistance, etc. Hence synergism does not acknowledge the doctrine of grace in the sense of Scripture and the Christian Church. On the contrary, it represents a return to the camp of Semi-Pelagianistic Romanism, which Luther and the Reformers so ceaselessly and zealously condemned. (Cp. Luther’s address to Erasmus in reply to his Diatribe: You alone have seen the key to things and sought the very throat.”) The Apology says Art. III, 144: "But works become conspicuous among men. Human reason naturally admires these, and because it sees only works, and does not understand or consider faith, it dreams accordingly that these works merit remission of sins and justify. This opinion of the Law inheres by nature in men’s minds;”
However, since synergism denies the monergism of divine grace (sola gratia), it really renders the conversion of a sinner impossible, since man is saved solely by grace, through faith, without the deeds of the Law, Rom. 3, 24—28; Eph. 2, 8. 9. Indeed, those who have been truly converted and have become children of God by trusting solely in Christ (sola fide) will fall from grace and lose their saving faith if they espouse the pernicious error of synergism. Gal. 5, 3.4.9.11.12.
Lastly, synergism involves its champions in hopeless contradictions and creates endless doctrinal confusion in all circles in which it is insisted upon; for even while it affirms man’s cooperation in conversion, that is, his own efforts as necessary for regeneration and salvation, it again stresses grace as the sinner’s only hope. Synergism is therefore an affirmation and a denial at the same time, a blending of grace and nature, which, consistently carried out, destroys “the central Christian truth of justification by grace alone and, with it, the assurance of a gracious God and of eternal salvation — the supreme religious concern of Luther and the entire Lutheran theology.” (Cp. Dr. F. Bente’s Historical Introduction on the Synergistic Controversy, Concordia Triglotta, p. 124 ff.)
Synergism derives its doctrine “not from any clear statements of the Bible, but by a process of anti-Scriptural and fallacious reasoning” ibid.; and it is all the more dangerous and pernicious since “it reduces man’s cooperation to a seemingly harmless minimum and clothes itself in ambiguous phrases and apparently pious and plausible formulas” ibid.. Its line of reasoning is: “Since all who are not converted or finally saved must blame, not God, but themselves for rejecting grace, those, too, who are converted must be credited with at least a small share in the work of their salvation, that is to say, with a better conduct toward grace than the conduct of those who are lost.” Ibid. This, however, in its final effect, overthrows the entire Gospel of free grace. It was for this reason that Luther and all confessional Lutherans so emphatically inculcated the monergism of divine grace. “The restoration of this wonderful truth, taught by St. Paul, made Luther the Reformer of the Church.” Ibid.
Also in its modern subtle form, synergism teaches that conversion is the product, in part, of man’s natural powers; for it is the unconverted sinner who must make the right use of the new powers granted to him by grace, decide in favor of conversion, cease wilful resistance, and the like. In its most subtle form, synergism makes conversion hinge on the non-occurrence of wilful resistance in a given moment or on a favorable condition and the consequent good attitude and conduct of natural man.
The theory that the Holy Ghost removes man’s natural resistance, but that the sinner himself must suppress all wilful resistance is Pelagianism pure and simple; for it ascribes spiritual powers to the unconverted. It is true, also dogmaticians like Gerhard, Quenstedt, Calov, etc., used the unscriptural expression that election took place intuitu fidei; but they repudiated the synergistic concept logically inherent in this phrase and taught that non-resistance is in no sense man’s own work, but rather the work of God, which unconverted man can only resist. (Cf. Dr. Pieper, Conversion and Election; Dr. Engelder, Dogmatical Notes.)