Taken from Col. G. L. Sicherman's walkthrough and Moogie and Sir Fil's Shining Force Page.

Name all characters: Start a new game and go to the "Name Your Character" screen. Press and hold Start, A, B, and C on controller 2 and press Start, A, and C on controller 1. Another character appears. Now you can cycle through and name every character in the game.

Message Test: You can also access a special "Message Test" simply by holding the B button at Simone's "Good Luck" message. Now you can read all of the game's dialog.

Whole game access: press Up and Start on the second joypad, then turn on the Genesis. Keep the buttons pressed until the Sega Logo comes up. Now press A and C (still holding down Up and Start) and press Start on the first joypad to begin the game. When Simone appears on the option screen, carry on from where you left off and press A, B or C. A will give you Chapter select. Button B gives you a Battle select and Button C will take you to the ending of the game.

Note: you can't actually save your game when you use this cheat, unless you statesave using an emulator.