I hold nothing sacred. Truth is always subjective and provisional. There are no absolutes.

Be true to yourself.
Happiness comes from within.
Never stop questioning.
Life is like riding a bike, if you stop peddling, you will fall off. (Albert Einstein)
How you say something is often more important than the words.
Time heals all wounds.
Consciousness is an illusion. (Sam Harris, Dan Dennett)
Don't spend your life looking for meaning in life, you won't find it.
We don't need religion to decide morality, Science if the best tool do that.

Along with others who have commented, I have to endorse the scientific method. Invaluable for supporting what's true and eliminating what's false.

Well . 22 years at ER I have learned that truth is hard to obtain , and , some humans live their lies as their reality .

I have learned to ask a lot of questions . I mean , really a lot .
And to look around for evidence . Physically and verbally .
I ask questions even about things that have 0 to do w the reasons of coming to ER .
I have learned that things are certainly not how it looks to be always , if ever ! I have learned that clothing , attitudes , julery , words are misleading . And I ask more questions .
Some times humans give in and share the real tragedy of their visit , their demons at home , their stress and their fears . Some times they rather hide as much as possible , but at least alarms are up and referrals can be given to take care minors or abusing situations .
Questions . That’s all I got . In my non working life , ( honestly , I don’t think I have one !), I question / read .

The Earth is sacred! People are sacred! Animals and Plants are sacred!! Everything is sacred and should be respected..

Scientific research

Research, study, reading, experience and education work for me. Learning from others. Counseling and self-reflection. Taking classes and workshops. Listening to people to increase my understanding, compassion and empathy.

1. Agreed -- Atheism is a statement of fact -- I see no evidence for gods so I don't believe gods exist..

2. Disagreed -- The etymology of the word, Sacred, is firmly mired in religion. Its synonyms are all words with religious overtones, its antonyms are all words with religious, overtones. A secular word to suggest a similar meaning might be something like "firm" or "resolute."

Math - the best place to find truth. and Logic - essentially verbal math.

Truth is determined via a preponderance of valid evidence. Very simple. Nothing sacred about it; it's just mandatory if you are interested in knowing (as best one can) what reality actually is.

question and research, and be prepared to change my opinion when they don't support it.

The scientific method is my method of finding the truth. I spent years studying both religion and atheism. I found no solid evidence supporting religion, and lots of evidence for atheism. Therefore, I am an atheist because I used the scientific method as my "methodology."

Well, when I was a manager faced with a complaint of some kind, the thing I always made sure that I did was talk with all parties involved to get their respective sides before taking any action. Not sure if this counts. It worked, for me at least, 9 times out of 10.

To be honest, I don't hold any Truths 'Sacred'.
However. The act of critical thinking, Questioning everything, and well, the Scientific Method. I hold above just about everything.
I'm not sure if I would Call any of them so called 'sacred", But they are important to me.
It may be That I just don't like the word.

My answer 

Methodology of finding the truth - The collective of cyclical processes known as science.

Maintained by : trying it out for improvements on every possible occasion where knowledge is required. It does not have to be in a laboratory. It can be with thought experiments in the mind.This makes it easier to alter conclusions.

The scientific method is our best way to try to discover the "truth." And, I suppose I think that this method should not be changed. Therefore, I guess you would say it is "sacred" to me--although I myself never use that word. This does not mean that errors cannot be made or that proposed hypotheses cannot be wrong (we are human after all); but, by holding true to the method the errors will be revealed.

"Seek Consonance" is both my highest truth, and my best method of finding truth. I don't expect it to be fulfilled to perfection, but the longer I follow it, the better my experience of life becomes. So the motivation to continue on that path is self-sustaining.

What kind of truth are you talking about?

Hard empirical truth does not give a shit what anyone thinks about it, it just is. Like a cancer growing in you does not care if you believe in Cancer or not, it just is.

Other uses of the term truth can be much more nuanced, and often are conflated with the above use of the term.
For instance it is true I like pizza, but if I develop lactose intolerance that might change. Nor is it true for all people.

and I agree with Mitch below.
sa·cred
/ˈsākrəd/
adjective

1. connected with God (or the gods) or dedicated to a religious purpose and so deserving veneration:

You are redefining the term to suit your op
"The word “ Sacred ” is not bound to be religious. It effectively means
“ Keep away,do not try to change this"
try
sac·ro·sanct
/ˈsakrōˌsaNGt/
adjective

1. (especially of a principle, place, or routine) regarded as too important or valuable to be interfered with:

the scientific method is the best way to get as close to the truth as our capacity can, but it is not sacred, actually what makes it effective is that it is not sacred, it can be attacked, patched, corrected and changed if you show that your change generates better results.
The power of statistics and reproducibility makes it auto-cleaning. Of course people are not perfect and there are abuses and people making (honest or not) mistakes. But once we see that there is a mistake, the method don't protect the "false knowledge" on the contrary, the method tells you to show the mistakes.

1. I agree with you. Atheism is non-belief in a deity. Nothing more.

2. I agree with you. Sacred means "leave this alone" like when I was raising my children, I tried to hold Sunday Dinner with all of us present, sacred. Emphasis on tried. I got my wish 10% of the time, but it was a goal.

I just have one, "Reality is Absolute" and there are no conflicts in reality.
So, following from this anytime things are in conflict, at least one of them is incorrect. All religious texts fall apart under this scrutiny.

Truth of Fact: Verifiable Evidience
Truth of the Heart: Kindness

Evidence is everything!

SCIENCE!
It works, bitches!