Engine will not stay running

Okay, just how many seconds will it run? Less than 3? If yes, you have a pass-lock system problem, Is the security light coming on then going off or is it staying on or flashing. If it isn't coming on and then going out as soon as you turn the key to start there is a hard failure in the system. A scan tool might show you an error if it can talk to the security system. It could be the pass-lock sensor, the tamper sensor or a failure in the body or power-train control module.
Did this just start happening out of the blue or did someone work on something and it started? What pass-lock does is interrupt the fuel supply by shutting the pump off if it sees a failure or wrong key or bad sensor. So you put the key in, turn it, the fuel pump runs the prime cycle and the engine starts, but the pump doesn't stay on.
Without a scan tool that can read the system any repairs will be a stab in the dark at best and some parts are expensive.