Legalism and Obedience

Some people consider Christians who actually act as The Bible teaches legalistic, is it really? The Bible lays down beliefs and actions that the Christian must present if that person is truly saved. Jesus says in Matthew 7:20 that we can recognize someone by their fruits, do they have bad fruit or good fruit? If someone goes back to sinning the way they were after being "saved", then that person may not be saved. We must know the difference.