For the way of all those who see is single and upward, illumined by the heavenly light, but the ways of those who do not see are many, dark and divergent; the one leads to the kingdom, uniting man to God, while the others lead down to death, separating from God. 
Saint Irenaeus’ On the Apostolic Preaching
Saint Irenaeus begins by addressing this work to Marcianus and stating that its aim is that he may “understand all the members of the body of truth” and “bear your salvation like fruit,” able to confound those who hold false opinions.
In the second paragraph he notes – against the Gnostics – the necessary unity of body and soul: “For these rejoice together and join forces to lead man to the presence of God.” In words that will be echoed by later Fathers, he insists on a link between holiness and knowledge.
And, in order to know the truth, it is necessary that we keep the rule (canon) of faith,
for faith is established upon things truly real, that we may believe what really is, as it is, and believing what really is, we may always keep our conviction of it firm. Since, then, the conserver of our salvation is faith, it is necessary to take great care of it, that we may have a true comprehension of what is.