For what reason is it necessary for us to finely tune an engine? The answer is simply that we have only a small range of ways in which matter can be arranged to produce a highly functioning engine. Fine tuning in necessitated by prior constraint. If I could do anything then there would be nothing to stop me from building an engine out of an amorphous lump of minerals. Physical constraint is the reason why I can’t do that.
What the fine tuning argument states is that there was a pre-existing constriction on God’s creation. That there was only a certain way he could arrange the universe in order to permit life. If we grant the fine tuning argument, then we must grant the following things: God could not have created the universe in any way he chose, in other words God is not omnipotent, and that a prior constraint in the laws of nature exists outside of God.
This creates problems because then one must ask where this law of nature came from that necessitated God’s fine tuning. What the fine tuning argument states is that some laws of physics existed prior to God. If there were no such laws, then there would be no parameters around which God would need to fine tune things, in which case the fine tuning argument would break down because without constraint, God could create a universe any which way he wanted, just as I could create an engine out of a lump of minerals. With constraint, one would have to ask from whence it came.