We have to take the whole section in, to understand the passage we are focusing on.

Christ has "canceled the charge of our legal indebtedness..." and "disarmed the powers and authorities, ..."
"Therefore..." the rest of our passage today.

"... do not let anyone judge you..."
Could be translated more clearly as "do not let anyone pass judgment on you..."

This is not to say no one can correct you or "be judgy". This is about not removing someone from fellowship in the body of Christ over things of the Law (which were dealt with by Christ on the cross).

"...what you eat or drink..."
Referring to Jewish kosher laws from the Old Testament and additional rabbinical teachings.

"...a religious festival, a New Moon celebration or a Sabbath day."
Jewish holy days: annual feasts, monthly celebrations, and weekly Sabbath.

The OT is not to be done away with... it is the shadow of Christ.

So where legalism was the first issue, now it's hyper-spiritualism and mysticism that is also a problem.

On false humility:

"Such a person also goes into great detail about what they have seen; they are puffed up with idle notions by their unspiritual mind."

They worship their own "visions" and are "puffed up" by their own imaginations. None of it is from God.

All prophecy and working of spiritual gifts (including teaching) must be in line with Christ as the head.

All true spiritual things must align with God and His word.

This is how we judge false teachers; false religions; and false prophets.

Be careful of rules added to scripture. True wisdom comes from the word of God, not humans.

The gospel is the good news that all who believe are reconciled to God through Christ and his sacrifice. Not through anything that we do.