One of the most consistent reasons I see people use against God being just and merciful is the story of God asking Abraham to sacrifice his son Isaac.
And it came to pass after these things, that God did prove Abraham, and said unto him: ‘Abraham’; and he said: ‘Here am I.’ And He said: ‘Take now thy son, thine only son, whom thou lovest, even Isaac, and get thee into the land of Moriah; and offer him there for a burnt-offering upon one of the mountains which I will tell thee of.’ And Abraham rose early in the morning, and saddled his ass, and took two of his young men with him, and Isaac his son; and he cleaved the wood for the burnt-offering, and rose up, and went unto the place of which God had told him. On the third day Abraham lifted up his eyes, and saw the place afar off. And Abraham said unto his young men: ‘Abide ye here with the ass, and I and the lad will go yonder; and we will worship, and come back to you.’ And Abraham took the wood of the burnt-offering, and laid it upon Isaac his son; and he took in his hand the fire and the knife; and they went both of them together. And Isaac spoke unto Abraham his father, and said: ‘My father.’ And he said: ‘Here am I, my son.’ And he said: ‘Behold the fire and the wood; but where is the lamb for a burnt-offering?’ And Abraham said: ‘God will provide Himself the lamb for a burnt-offering, my son.’ So they went both of them together. And they came to the place which God had told him of; and Abraham built the altar there, and laid the wood in order, and bound Isaac his son, and laid him on the altar, upon the wood. And Abraham stretched forth his hand, and took the knife to slay his son. And the angel of the LORD called unto him out of heaven, and said: ‘Abraham, Abraham.’ And he said: ‘Here am I.’ And he said: ‘Lay not thy hand upon the lad, neither do thou any thing unto him; for now I know that thou art a God-fearing man, seeing thou hast not withheld thy son, thine only son, from Me.’ And Abraham lifted up his eyes, and looked, and behold behind him a ram caught in the thicket by his horns. And Abraham went and took the ram, and offered him up for a burnt-offering in the stead of his son. (Gen 22:1-13)
I took this section from the Jewish Publication Society Bible.
How can a God who says you shall not sacrifice a child ask a man to sacrifice his son? God seems like a liar and one who contradicts himself.
One of the misconceptions is that Isaac was a child. We was just born a little while ago and now he is a young child. The problem with that is rabbinic tradition does not hold to that view. In fact, rabbis have Isaac being as old as 37 when he was bound by Abraham. That is a far cry from a scared child being sacrificed by a delusional father. How can a 37 year old man be tied up against his will by a 137 year old man? Isaac would have to have been a willing sacrifice.