"The formation of the life of a person in the womb is the work of God, and it is not merely a mechanical process but a work on the analogy of weaving or knitting: "Thou didst knit me together in my mother's womb" (psalm 139:13). The life of the unborn is the knitting of God, and what He is knitting is a human being in His own image, unlike any other creature in the universe... The destruction of conceived human life - whether embryonic, fetal, or viable - is an assault on the unique person-forming work of God."

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It is evil to justify killing (unborn babies) by the happy outcome of eternity for the one killed. This same justification could be used to justify killing one-year olds, or any heaven-bound believer for that matter. The Bible asks the question: "Shall we sin that grace may abound?" (Romans 6:1) And: "Shall we do evil that good may come?" (Romans 3:8). In both cases the answer is a resounding NO. It is presumption to step into God's place and try to make the assignments to heaven or to hell. Our duty is to obey God, not to play God.

— John Piper