Paul of Tarsus, like Jews and Roman pagans of his time, believed that sacrifice can bring about new kinship ties, purity, and eternal life.For Paul the necessary sacrifice was the death of Jesus: Gentiles who are "Christ's" are, like Israel, descendants of Abraham and "heirs according to the promise". The God who raised Jesus from the dead would also give new life to the "mortal bodies" of Gentile Christians, who had become with Israel the "children of God" and were therefore no longer "in the flesh".Modern Christian churches tend to be much more concerned with how humanity can be saved from a universal condition of sin and death than the question of how both Jews and Gentiles can be in God's family. According to both Protestant and Catholic doctrine, salvation comes by Jesus' substitutionary death and resurrection. Martin Luther taught that baptism was necessary for salvation, but modern Lutherans and other Protestants tend to teach that salvation is a gift that comes to an individual by God's grace, sometimes defined as "unmerited favor", even apart from baptism. The Catholic Church teaches that salvation does not occur without faithfulness on the part of Christians; converts must live in accordance with principles of love and ordinarily must be baptized.Christians differ in their views on the extent to which individuals' salvation is pre-ordained by God. Reformed theology places distinctive emphasis on grace by teaching that individuals are completely incapable of self-redemption, but that sanctifying grace is irresistible. In contrast Catholics, Orthodox Christians and Arminian Protestants believe that the exercise of free will is necessary to have faith in Jesus.