"
The door is locked now, but it's open if you're true
The door represents himself, his heart, who he is. He's telling her if she's real with him, loves him, cares for him, gives him a reason to trust her, and won't harm or betray him, that he will let her into his heart and world. He will open himself up to her; or open the "door". Later on it talks about a key. We can infer that the key is the one that opens the door or himself. We aren't talking about a literal door or key. When you lock your door you don't want anyone to come in, you're keeping things out, keeping yourself in and isolated.