Saint Augustine of Hippo was a prominent theologian and philosopher in the 4th-5th centuries AD who helped shape many Christian doctrines. He is renowned for clarifying the Trinity, predestination, original sin, grace, and other theological concepts. Augustine emphasized that humanity is incapable of achieving salvation through free will alone and requires God's grace. He argued against the Pelagian idea that humans can attain salvation through their own moral efforts without divine aid. Augustine maintained that God's grace is necessary to overcome the effects of original sin and enable humans to have free will and do good. His writings, such as Confessions and City of God, had immense influence on Western Christianity.