An epiphany—a manifestation of the divine or a sudden perception of a thing’s essential meaning or nature—calls for response, reaction.
Of all the epiphanies revealing Christ to mankind, St. Paul focuses on the ultimate unifying epiphany…the Cross of Christ!
For Paul, the Cross of Christ is the central truth of our faith that enlightens the human mind. To speak "only of Christ crucified" means that every issue is understood by the cross. We decide to know nothing but the cross because the cross enlightens everything.