It's a scam. Essentially a variation of the Nigerian check scam.
Once you deposit the phony check, they usually ask you to send some amount back to them, for "processing fees" or "taxes", possibly thru Western Union so you can't reverse it or track them down.
But just because the bank deposits money in your account and releases some or all of it to you does not mean the whole transaction cannot be reversed if it turns out the check is bad or forged. Your bank does not guarantee some other unknown party's check is good. In fact, you are on the hook to the bank if it is not.
Since most banks will release part of a deposited check quickly if you have been a reliable customer, you can end up sending your real money (as "fees" or "taxes") to crooks, thinking you got more than that from them in the check. It can take several weeks for the bank to find out the check is phony, and even months to find out if it is forged on a legitimate account. Regardless of how long it takes, the bank reverses the deposit and you end up responsible for any amount short.
If you didn't actually send them any money, then although they tried to take you, you have no real loss, other than being fooled into believing you have money you don't really have and possibly spending it on things you cannot afford.
If that is all you lost, consider yourself very lucky. Some victims of this type of scam have found themselves responsible for returning thousands of dollars to their bank, and in some cases even had legal costs to defend against charges of depositing forged or fraudulent checks, while they sent their own real hard earned cash to the crooks.
Note that scammers steal the legitimacy of others (in this case the credit union), as well as make other fraudulent claims ("the money is in an insured account") to make it all sound legitimate and low risk. Their goal is to maintain the fiction long enough to get you to part with YOUR money, either thru you sending it to them via Western Union, or thru you giving them your bank account information.
Just because people say something doesn't make it true, and just because they put something on a piece of paper (a check), doesn't make it real.