Two quotes that answer your question better than I ever could:
One is, I think, at least one source of the concept:
“I know your deeds; I know you are neither hot nor cold. How I wish you were one or the other–hot or cold! But because you are lukewarm, neither hot nor cold, I will spew you out of my mouth! You keep saying, “I am so rich and secure that I want for nothing.” Little do you realize how wretched you are, how pitiable and poor, how blind and naked!” Rev. 3:15-17
Then there is this more recent one, from a very well-known Trappist monk and convert to Catholicism:
“There is no neutrality between gratitude and ingratitude. Those who are not grateful soon begin to complain of everything. Those who do not love, hate. In the spiritual life, there is no such thing as an indifference to love or hate. That is why tepidity (which seems to be indifferent) is so detestable. It is hate disguised as love.”
Thomas Merton, from Chapter VII, Thoughts in Solitude
(my copy was published in 1968 with Nihil Obstat, Imprimi Potest, and Imprimatur… which is to say it has been declared free of doctrinal or moral error, not that everyone has to agree with it.)