The Reformed admit a true power of not converting, alongside efficacious grace
They do maintain that once the grace by which God converts men is given, conversion cannot fail to follow, and that the motion and push of converting grace cannot coexist with a resistance of the human will strong enough to block the act of conversion.
Still they recognize that, together with the efficacy of grace, a power remains for the opposite act and for not performing the act grace drives a man to. A man who is converted infallibly still always keeps a real power not to turn himself and to produce the contrary act.
Latin
XLIV. Quinimo quamvis pertendant, posita gratia per quam Deus homines convertit, fieri non posse quin conversio sequatur; nec simul stare posse gratiae convertentis motum atque impulsum, & talem voluntatis humanae resistentiam, quae conversionis actum impediat: attamen agnoscunt simul cum efficacia gratiae consistere potentiam ad actum oppositum, & negationem actus ad quem gratia hominem impellit. Nempe hominem, etsi convertatur infallibiliter, semper retinere veram potentiam se non convertendi, & contrarium actum eliciendi.