It is, however, doubtful whether any of today's deniers of penal substitution could match the malevolent impact made by Faustus Socinus' De Jesu Christo Servatore (written in 1578, published in 1594). The work can be read in Dutch and Latin but has never been translated into English. Packer has the following comment on it in his classic 1973 lecture What did the cross achieve? The logic of penal substitution (if you have never read this brilliant piece it is available here):
What the Reformers did was to redefine satisfactio (satisfaction), the main medieval category for thought about the cross. Anselm’s Cur Deus Homo?, which largely determined the mediaeval development, saw Christ’s satisfactio for our sins as the offering of compensation or damages for dishonour done, but the Reformers saw it as the undergoing of vicarious punishment (poena) to meet the claims on us of God’s holy law and wrath (i.e. his punitive justice).Many of the arguments put forward by Socinus have resurfaced again and again in history when penal substitution has been under attack. Of Socinus' book Owen said that it was written with the greatest strength, subtilty, and plausibility of all of the productions that came from him and his followers.
What Socinus did was to arraign this idea as irrational, incoherent, immoral and impossible. Giving pardon, he argued, does not square with taking satisfaction, nor does the transferring of punishment from the guilty to the innocent square with justice; nor is the temporary death of one a true substitute for the eternal death of many; and a perfect substitutionary satisfaction, could such a thing be, would necessarily confer on us unlimited permission to continue in sin.
Socinus’ alternative account of New Testament soteriology, based on the axiom that God forgives without requiring any satisfaction save the repentance which makes us forgivable, was evasive and unconvincing, and had little influence. But his classic critique proved momentous: it held the attention of all exponents of the Reformation view for more than a century, and created a tradition of rationalistic prejudice against that view which has effectively shaped debate about it right down to our own day.
Owen's counsel on the defense of the atonement is worth pondering:
I dare boldly acquaint the younger students in these weighty points of the religion of Jesus Christ, that the truth of this one particular, concerning the eternal justice of God indispensably requiring the punishment of sin, being well established...will securely carry them through all the sophisms of their adversaries, and cut all the knots which, with so much subtilty, they endeavour to tie and cast upon the doctrine of the satisfaction of Christ.John Owen, Vindicae Evangelicae, p. 28