Angels are basically classified as either righteous or evil. The former retain their holy state and service to God and are called elect angels (1 Tim 5:21), whereas the latter have defected from their original state and continue in constant rebellion against God. Satan is the leader of all fallen angels (Matt 25:41; cf. Rev 12:7, 9), which Scripture designates as evil spirits (1 Sam 16:14; Luke 7:21), demons (Matt 8:31), and unclean spirits (Mark 5:1-4). These have been operating for millennia trying to frustrate the purposes of God.