Firstly, you need to understand that the way God dealt with people under the old testament (or old covenant) is very different to the new.
Many things that happened back then could not happen now. For example, judgement. God is not going to strike people dead under the new covenant.
Why? because ALL judgement has been given to the son
"For the Father judges no man, but has committed all judgment unto the
Son" - John 5:22.
Jesus will one day return and Judge as is revealed in the book of Revelation. But, he hasn't come back yet. So until that time, you're not going to be struck down for making a mistake.
So looking at the character of Jesus (who represents the Father), how does he deal with people? He loves them.
Here is an example of the sort of thing that happened in the old testament, but with a very different outcome in the new.
54 And when his disciples James and John saw this, they said, Lord,
will you that we command fire to come down from heaven, and consume
them, even as Elijah did? 55 But he turned, and rebuked them, and said, You know not what manner of spirit you are of. 56 For the Son of man is not come to destroy men's lives, but to save them. And they went to another village. - Luke 9:54 - 56
I know some people will look at what happened in Acts 5:9 and say God judged Ananias and Sapphira and try to rebut what I'm saying here. But if you read acts 5 more carefully no where does it say that God struck them dead. And if what happened to them was God then you must ask yourself why God isn't killing Christians for lying. I would say it's because it wasn't God who killed them. If he was that harsh there wouldn't be any Christians to preach the Gospel and bring the good news! Neither would he say
"My little children, these things write I unto you, that you sin not.
And if any man sin, we have an advocate with the Father, Jesus Christ
the righteous:" 1 John 1
God is not dealing with people like his was in the old testament. And the reason is because of Jesus. Jesus bore ALL of our sin at the cross. Now, people can be born again and made spiritually alive. They can be set free.
So if some guy in 2013 claims that God told him to kill his son, you know for sure that wasn't an instruction from God. God has not come to destroy men's lives but to save them.
I should further add that many of the events that occurred in the old testament were shadows (or images) of the Death/Burial and Resurrection of Jesus including Abraham being put to the test. Can you see it? It'll never happen again, we don't need another shadow when we have the reality in Jesus. :-)