How do christians explain why their god had to sacrifice his son in order to save people. Is god not powerful enough to save whoever...?

Bloodthirsty god's of ancient imagination are like that.

It made sense to ancient barbaric minds:

"mongo need innocent blood to like little human pets again..."

God can forgive whomever he wishes, but he would have to do so at the expense of his perfect justice. And that is something he will not do. Not cannot do, but will not.

Jesus asked that if there were any other way to save us, that it be used.

Matthew 26:39 :And he went a little further, and fell on his face, and prayed, saying, O my Father, if it be possible, let this cup pass from me: nevertheless not as I will, but as thou wilt."

God would have answered Jesus' prayer if the WERE any other way. As he did not, there was no other way to accomplish this.

4 When he created Adam, Jehovah gave him something truly precious—perfect human life. Consider what that meant for Adam. Made with a perfect body and mind, he would never get sick, grow old, or die. As a perfect human, he had a special relationship with Jehovah. The Bible says that Adam was a “son of God.” (Luke 3:38) So Adam enjoyed a close relationship with Jehovah God, like that of a son with a loving father. Jehovah communicated with his earthly son, giving Adam satisfying assignments of work and letting him know what was expected of him.—Genesis 1:28-30; 2:16, 17.

5 Adam was made “in God’s image.” (Genesis 1:27) That did not mean that Adam resembled God in appearance. As we learned in Chapter 1 of this book, Jehovah is an invisible spirit. (John 4:24) So Jehovah does not have a body of flesh and blood. Being made in God’s image meant that Adam was created with qualities like those of God, including love, wisdom, justice, and power. Adam was like his Father in another important way in that he possessed free will. Hence, Adam was not like a machine that can perform only what it is designed or programmed to do. Instead, he could make personal decisions, choosing between right and wrong. If he had chosen to obey God, he would have lived forever in Paradise on earth.

6 Clearly, then, when Adam disobeyed God and was condemned to death, he paid a very high price. His sin cost him his perfect human life with all its blessings. (Genesis 3:17-19) Sadly, Adam lost this precious life not only for himself but also for his future offspring. God’s Word says: “Through one man [Adam] sin entered into the world and death through sin, and thus death spread to all men because they had all sinned.” (Romans 5:12) Yes, all of us have inherited sin from Adam. Hence, the Bible says that he “sold” himself and his offspring into slavery to sin and death. (Romans 7:14) There was no hope for Adam or Eve because they willfully chose to disobey God. But what about their offspring, including us?

7 Jehovah came to mankind’s rescue by means of the ransom. What is a ransom? The idea of a ransom basically involves two things. First, a ransom is the price paid to bring about a release or to buy something back. It might be compared to the price paid for the release of a prisoner of war. Second, a ransom is the price that covers, or pays, the cost of something. It is similar to the price paid to cover the damages caused by an injury. For example, if a person causes an accident, he would have to pay an amount that fully corresponds to, or equals, the value of what was damaged.

Get one thing straight. God does not HAVE to do anything. He offered up his only begotten Son for the sins of mankind as an expression of his love for his creation. And Jesus submitted to and obeyed his Father out of that same kind of love.

Jhn 3:16 For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.

Jhn 3:17 For God sent not his Son into the world to condemn the world; but that the world through him might be saved.

Jhn 15:13 Greater love hath no man than this, that a man lay down his life for his friends.

God is perfect, there is NO sin in Him. Sin requires a death penalty that MUST be paid. God is a just judge, therefore SOMEONE must pay the death penalty, and Jesus, God's Son volunteered to pay that penalty for everyone who will accept the free gift, through Grace. Take it, or leave it (then YOU pay your penalty of death YOURSELF, but if you ONLY accept the free gift of the life of JESUS, then you will NOT have to pay the death penalty yourself ---- Jesus has paid it for you). Just don't be silly, take the offer while you can.

God is a God of law. Without it, all things would be meaningless, including God. The law demands a punishment for breaking of it, but though the sacrifice of Jesus, who paid the price of the law, that there is also mercy to those who accept his sacrifice. Both the demands of justice and mercy are satisfied.

God follows His own Laws. Sin must be punished. Jesus took the punishment for us. That's it. We are saved because of the Lamb of God. God has three natures. Son of God died to restore our relationship with the Father of God. Sin is punishable by death. God is all powerful, but He follows His own laws.

He follows the laws of love and common sense. We don't live in an animal sacrifice culture so we don't have to explain why the Father had to allow his Son to be sacrificed. The symbolism is lost on us today, that's why we concentrate on the other, real aspects of Jesus' sacrifice. My guess is that you do not really want to know, nor do you care.

God as merciful he is Just

They don't understand the problems with the scenario. They just repeat the answers they were taught in church indefinitely.