After a refurbished ECM was installed the car will not start?

I installed a refurbished ECM that I purchased, and it comes programmed and with a key. I followed instructions on how to install it and program the key to the car. The car won't start! The locksmith that came to my house to cut the new key told me that the anti-theft might have kicked on. Which it did but the new key won't help anything, but the old keys can. With the old key I got the anti-theft lock off but as soon as I try to use the new key it kicks back on. I put the old ECM back in my car and it turns right on. Someone told me I probably need to get my ECM and key programmed again. This isn't cheap so I need a more certain answer. Please help! TIA