Emergent Church and Post Modernism: "Truth cannot be known with Certainty"

One of the core doctrines of the Emergent Church is the philosophy of alleged postmodernism with its assertion that truth cannot be known with certainty. But here is what the Lord says of His true sheep:

"Now they have come to know that everything You have given Me is from You; for the Words which You gave Me I have given to them; and they received them and truly understood that I came forth from You, and they believed that You sent Me." (John 17:7-8)