"How so? To leave the faith means not to hold it anymore."
…because your original comment is a fallacy of composition, namely this one:
"No longer holding the faith and leaving the faith seem synonymous to m me when speaking like this."
Promoting one error does not imply renouncing ALL Catholic teachings in favor of other errors.
"If you no longer hold to the infallible teachings of the Church, you have departed from the church,"
No, at the least you are in error, at the worst (following ignored correction by The Church) you are a heretic. So long as you retain other (critical) teachings, you haven't left The Church. Further, doing so implies a positive act, which Cdl. Marx has not in any way done.
"I'm not going to get into the merits of the argument of when one no longer counts as holding the faith."
The faith includes more than The Church's teaching on homosexualty. Preaching one error does constitute leaving The Church or no longer holding the faith, in toto.
"then it's a little more than just preaching error."
Technically, no. Until The Church steps in, it's still just an error. At least according to Church Law and The Church's definition of heresy. Cardinal Marx isn't anywhere near the standards for apostasy which really WOULD entail "having left the Catholic faith".
"This is seemingly part of the "legalistic" approach. You are taking "leaving" as literally leaving."
Heaven forbid a reader should take the word "leave" to literally mean "leave"! :D Or assume Bp. Strickland undestands the meaning of "leave" or "the Catholic faith", eh?
Apparently in Texas, "has left the Catholic faith" DOESN'T mean he "left the Catholic faith". It means something ELSE. Goodness! Let's apply that generously apologetic interpretation of "eave" to Cardinal Marx and his use of the word "sin"!
When Cardinal Marx says homosexuality "is not a sin", he means something ELSE , even though he used the word sin. He probably meant something else like "it's not a good idea" or something ELSE that doesn't mean sin. :P
"But the quote I provided shows that one no longer holds to the faith if they reject just one part of it.
Not so. The faith includes more than one teaching.
"On the contrary, Just as mortal sin is contrary to charity, so is disbelief in one article of faith contrary to faith."
Contrary to faith and leaving the Catholic faith are not the same. The former is a contrast against what the Church teaches, the latter an act of leaving the religion and all its beliefs entirely.
"If a man willfully rejects one article of the faith, faith no longer remains in him."
Meaning faith (i.e. belief) in the faith (The Catholic church and its teachings), no longer remains in him.
Belief (faith) and a religion (the faith) are not the same.
""otherwise, if, of the things taught by the Church, he holds what he chooses to hold, and rejects what he chooses to reject, he no longer adheres to the teaching of the Church as to an infallible rule, but to his own will."
Irrelevant to him leaving The Church or the faith outright. Caffeteria Catholicism is a rampant problem among Catholics. They haven't all "left the Catholic faith" though as Bp. Strickland would claim based on your quote.