Among Christianity’s many influences on Western civilization is the concept that “all men are created equal.” The idea that human beings are equally valuable simply by virtue of being human is common even among those who otherwise reject religious teachings. What was once solely a religious principle has become almost universally accepted as a humanist principle. Two people might differ in their race, gender, intellectual abilities, age, or level of wealth, but both are equally deserving of respect, fair treatment, and equality under the law.
Call this “Belief A”.
Most people who support abortion argue that the fetus gradually acquires human rights as his/her nervous system develops in utero. According to polling done by Gallup, 61% of Americans believe that abortion should be legal during the first trimester of pregnancy. That figure drops to 27% during the second trimester, and to 14% during the third trimester. Since human life biologically begins at conception, this means that roughly half of Americans believe that some human organisms (third trimester fetuses) should have a legal right to life, while other human organisms (first trimester fetuses) may be legally dismembered with metal clamps and vacuum hoses.
In other words, these individuals believe that human worth is not derived “simply by virtue of being human,” but rather by possessing certain physical qualities. Thus, all humans are equal, but some humans are more equal than others. Call this “Belief B”.
A person cannot rationally hold both Belief A and Belief B. If “social justice” extends to some groups of humans – but not to others – then it isn’t really social justice.