The main idea of this quotation is that the things that we create, the things we do, what we learn, and they things we worship shape who we are as human beings. The quotation is telling us that things like religion, education, and art and music is what separates humans from animals, it is what makes us humans, or 'have a soul'. We can understand this because Thomas More describes "The many great gardens of the world" as art, music, religion, and education, and right after states "The soul cannot thrive in the absence of a garden" This garden represents the art, music, religion, and education. He goes onto to if one does not have a soul, one is not human, and we cannot have a soul without having these "gardens" or things that make us human.
Hope this helps! :)