Only God has the right to end life.
The decision to take a life is God's alone to make. Any instance of a human taking another human's life is immoral and wrong. 
To kill an unborn child, which God created, is to disrespect the sanctity of human life. It is to express an arrogance to presume that you have the right to decide matters of life and death, which lies exclusively in the remit of God.
This argument holds weight in countries where the legal system is linked to a religious entity. In the Islamic Republic of Iran, where the state is based on its interpretation of Islamic laws, this view of abortion is understandable, for example. However, in most Western societies, our government prides itself on its secular philosophy. The modern secular society has a state free from religious influence and considers religious institutions as an entirely separate institution. Therefore, to prohibit abortion on religious grounds would be a violation of Western democracy's secular principles. It would be an example of religious ideology influencing matters of government, a practice most Western societies abandoned during the Age of Enlightenment.
[P1] Only God can give life or take it away. [P2] Therefore, taking another human's life is immoral. [P3] Therefore, abortion is immoral. [P4] Therefore, abortion should be illegal.
[Rejecting P1] This religious ideology has no place in dictating government policy in secular society.