As children of God, made in His image, we are to be His representatives on this earth. Just as He defends those who cannot defend themselves, so must we. We have an ironclad obligation to speak up when we see those who are "dumb," meaning those who have no voice for themselves. These could be unborn babies, children, the disabled, older adults, or animals. We have a voice compelled to use it. When we see someone being bullied or mistreated in any way, we are to speak up. We are called to defend the defenseless, just as God defends us.
