All of your life, you have respected the police. You know they are in a position of authority. You are polite to them when you encounter them. You don't go out of your way to cause trouble.
However, you start to reconsider your position when an officer shows up at your house, knocks on the door, and asks if he can come in and take a look around. Do you have to let the police in? Should you?
Whether or not you should depends on the situation, but you should remember that you do not have to. It's your home. You do not have to give in if the police ask to come in. You don't have to take your respect for authority that far. They need to respect you.
There are exceptions, of course. If the police have a warrant that says they can enter the home, they can do it even if you tell them not to. They typically need to show you the warrant, though. Ask to see it if they claim to have one.
In some rare cases, they can enter without a warrant. They need probable cause. If they claim to have it -- they think a crime is happening at the moment, for instance -- remember that they will need to show that probable cause in court. It's usually far better for them to use the probable cause to get a warrant in advance, but they don't always do so, especially when speed is important.
If the police violate your rights while arresting you or at any point, make sure that your defense attorney knows that. It may be an important factor in your case.