Why did God ask Abraham to sacrifice his Son? And why did he say Abraham's ONLY son? Ishmael was also a son of Abraham. Isaac wasn't even the first born. So, I'd like to see responses by Jews, Muslims and Christians on this.

Giving Your Firstborn Son to God - TheTorah.com
Exodus 22:28 commands Israel to give its firstborn sons to God, and makes no mention of redeeming them. What exactly is being commanded? | Dr. Eve Levavi Feinstein

Exod 22:28 You shall not put off the skimming of the first yield of your vats.[1] You shall give Me the firstborn among your sons. 22:29 You shall do the same with your cattle and your flocks: seven days it shall remain with its mother; on the eighth day you shall give it to Me.