Trust and truth
In my opinion there are a few things problematic about the sentence “Always telling the truth is the most important thing in a relationship.”
Frist of all “always” is a very strong word especially if it comes to lies. Little white lies are part of our everyday life and make it easier. Sometimes little lies are the right thing to tell. For example: If I know a friend really needs my help and (s)he asks if I have time my answer will be yes, even if that is technical a lie.
Second “the most important thing in a relationship” is very hard to define. It is different from person to person. For me trust is the most important thing. How the truth relates to trust I will discuss in the final part of my discussion.
Third the definition of the word “relationship”. There are very different kinds of relationships, for example lovers, friends or family. Sometimes lies are part of a relationship. For example parents ‘lie’ to their kids about the Easter bunny.
All in all I think you should say that trust is the most important thing in a relationship. Lies are sometimes necessary but trust is the feeling we wish for when we talk about telling the truth.
I need to trust a person to know when to the truth is needed and when it is time for a little lie. I may even forgive a lie where the truth should have been spoken if I understand and accept the reasons for the lie.