I Pity the Fool
I pity all fools!
...but most especially Gnostic fools! Irenaeus was known affectionately as the "Lion of Lyons," since one of the main cities he preached in was what is today Lyons, France. Gnosticism taught that there were many secrets only known to a select few, and had many heretical beliefs, or at least beliefs that were in opposition or were distortions of true Catholic dogma. (One such belief is that Judas was one who knew 'the secret,' and thus fulfilled God's mission for him in betraying Jesus!).
Gnosticism still holds much sway in the anti-Catholic circles, as there are many who either blatantly deny history, or try bending things in such a way as to distort the truth to their likings. Irenaeus would have something to say to them!
Irenaeus was also adamant about unity throughout the Church, insisting on the primacy of the episcopacy (Bishops), and stressing the Church's tradition and Scripture as the two fonts of truth and unity.
Read more about Irenaeus' work in the Catholic Encyclopedia. (His feast day is today! July 3).