Let us remember that this poem was written in response to new laws brought in by the British parliament against the freedom of the press and trying to censor public speech. Milton's impassioned defence of free speech is the nature of this poem, and in it he refers to truth in many ways.
One central reference to truth comes in his fourth argument, when he argues that censoring publications has a profounly negative impact on the truth, both in terms of the truth that England already has but also thinking about the discovery of new truths. For Milton, truth must never be allowed to sit still without having the opportunity to grow and develop. This means that any "truths" have to be open to the rigour of public debate and criticism in order for them to be refined. Truth should never be a matter of mere unthinking acceptance.
For Milton, therefore, censorship is the enemy of discovering new truths. He argues that truth is a gift that is given to those who seek it only in order to achieve ever greater heights of wisdom. England has the moral responsibility to search "what we know not by what we know," as Milton put it. He went further by arguing that the recent discoveries and period of learning experienced by England is a sure sign of divine favour, pointing towards some kind of new golden age in terms of discovery and the truth. The truth is therefore something to be sought and discovered, and man fulfils his purpose when he sets himself on a quest devoted to its search.