"Areopagitica" (1644) was written to argue against the Licensing Order of the previous year, which required that works had to be pre-approved by a censor before they could be printed. Some historical background is necessary to fully understand the passage. Until 1640, the Catholic monarch Charles I had claimed to be above parliament. During his absolutist rule, he had mandated that his Star Chamber would pre-censor documents before allowing them to be legally printed. By 1642, Parliamentarians had managed to place curbs on Charles's power. However, in 1643, Parliament too passed a similar licensing order requiring pre-censorship. Milton's essay is addressed to Parliament, urging them to reconsider.
Milton argues that true freedom consists not in being free from grievances, but being free to air them. He says that England has come very close to achieving true freedom. He says that his being able to circulate a pamphlet arguing against the government proposal itself demonstrates this true freedom that prevails in England: To which if I now manifest by the very sound of this which I shall utter, that we are already in good part arrived.
He goes on to say that this freedom was hard-won, because it had to contend against such a steep disadvantage of tyranny and superstition grounded into our principles as was beyond the manhood of a Roman recovery. This refers to the underpinnings of Charles's rule: the more or less absolute monarchy that he insisted upon (tyranny) and his Catholic faith (superstition). The overthrow of his personal rule and reinstatement of Parliamentary checks on his power had led to a civil war that was still underway as Milton was writing. Under Charles's absolute rule, liberty had been impossible. Publishing an anti-government tract would have been considered an act of treason. That Milton was now able to do so showed that with great difficulty, tyranny and superstition had finally been overthrown.
So deep was the disadvantage this tyranny and superstition placed upon champions of liberty, that it was beyond the manhood of a Roman recovery. The term manhood is a literal rendering of virtue, which traces back to man. The Online Etymological Dictionary says:
c. 1200, vertu, "moral life and conduct; a particular moral excellence," from Anglo-French and Old French vertu "force, strength, vigor; moral strength; qualities, abilities" (10c. in Old French), from Latin virtutem (nominative virtus) "moral strength, high character, goodness; manliness; valor, bravery, courage (in war); excellence, worth," from vir "man".
Milton says that the typical Roman private virtues of moral excellence, strength, ability, etc. had not been enough for liberty to prevail under the tyranny and superstition of Charles's rule. Only God's deliverance allowed liberty to finally flourish. Milton flatteringly adds that the Parliamentarians to whom his tract is addressed were as responsible as God for this happy state of affairs as well.
Roman recovery is double-edged, though. On the one hand, it does literally mean that the Roman manhood, human and specifically male virtue, was insufficient; God's grace was needed to restore liberty. On the other hand, Roman is also a broadside against Charles's Catholicism. Manhood/virtue by itself was not enough to recover liberty from the constraints placed on it by the tyranny and superstition of Roman Catholicism. This is also a theological argument. Unlike Catholicism, where adherence to the sacraments of the Church are sufficient for salvation, in Milton's Protestant theology, salvation and deliverance are in the hands of God and not determined by individual virtue. The association of virtue with manhood also has connotations of the etymologically related virility. Milton is casting the Protestant/Catholic distinction in gendered terms, with the former being masculine and the latter feminine.
Milton has a sophisticated conception of liberty, and his complex rhetoric here brings that out. Liberty does not entail doing what one wants without opposition; rather, it means having the freedom to dissent. Liberty is not a natural condition; to achieve liberty, one has to actively fight against forces of tyranny and superstition. The fight requires manliness, but such virtue is not sufficient; God's deliverance is necessary as well. Milton contrasts the Roman Catholicism of Charles's regime with the Puritanism of the parliamentarianists, and cautions them against replicating the same error of placing undue constraints on liberty.