Thank you for being caring and brave in asking this difficult question.
While we know that salvation is based on faith (not based on works or being good), there is still some comfort we can give to your friend about his father.
First, we know that unfortunately many Jewish people do not get a chance to hear the gospel presented in a way that is attractive to them. And in many cases, it is not the fault of the Jewish person.
Throughout history, there has been a lot of antisemitism and forced conversions. During the Spanish Inquisition, for example, many Jews were forced to convert to Christianity or faced with death. Even after they converted, they were targeted for suspected practice of Judaism. During the Holocaust, many so-called "Christians" sided with Hitler's plans in condemning and persecuting Jewish people.
Such actions are very antichrist-like.
But Jesus is Jewish and so were the earliest disciples. The Bible was penned by Jews.
"Then what advantage has the Jew? ... Great in every respect. First of all, that they were entrusted with the oracles of God." (Romans 3:1-2)
"...for salvation is from the Jews." (John 4:22)
But many professing "Christians" did harm to Jewish people, causing them to have an aversion to Christianity. Conversion to Christianity is seen as a betrayal of a Jewish person's family, heritage, and country.
This is probably the primary reason your friend's father might not have become a Christian while he was on earth.
However, there is hope.
We know that God judges righteously and gives everyone a chance to believe.
“Shall not the Judge of all the earth do what is just?” (Genesis 18:25)
Jesus said, “If I had not come and spoken to them, they would not have sin...” (John 15:22).
People are not responsible for what they have not heard. But they are responsible for what they do hear.
It is likely your friend's father did not get a chance to hear the gospel. Or your friend's father did not get a chance to hear the gospel presented in a way that addressed his deeply rooted concerns.
If this is true, then he would be given another chance even though he is not physically alive anymore.
The Bible tells us:
“They will give account to Him who is ready to judge the living and the dead. For the gospel has for this purpose been preached even to those who are dead, that though they are judged in the flesh as men, they may live in the spirit according to the will of God.” (1 Peter 4:5-6)
This verse gives us a clue as to what happens to those who never heard.
During the first century AD, Jesus' disciples tried hard to spread the gospel as quickly as possible. But it was not possible to reach everyone. Even today, we have not preached the gospel to every human on earth. Then what about those who never got a chance to hear?
1 Peter 4:5-6 tells us that those who never heard would get a chance even after they die. Perhaps angels preach the gospel to them.
So maybe your friend's father heard the gospel posthumously.
It would be up to your friend's father to accept or reject it.
But God on His part would not judge until your friend's father got a fair chance.
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The prophet Ezekiel saw a valley with dry bones that came alive.
"He said to me, 'Son of man, can these bones live?' And I answered, 'O Lord God, You know.' Again He said to me, 'Prophesy over these bones and say to them, "O dry bones, hear the word of the Lord." Thus says the Lord God to these bones, "Behold, I will cause breath to enter you that you may come to life. I will put sinews on you, make flesh grow back on you, cover you with skin and put breath in you that you may come alive; and you will know that I am the Lord."
Then He said to me, 'Son of man, these bones are the whole house of Israel; behold, they say, "Our bones are dried up and our hope has perished. We are completely cut off." Therefore prophesy and say to them, "Thus says the Lord God, 'Behold, I will open your graves and cause you to come up out of your graves, My people; and I will bring you into the land of Israel. Then you will know that I am the Lord, when I have opened your graves and caused you to come up out of your graves, My people. I will put My Spirit within you and you will come to life, and I will place you on your own land. Then you will know that I, the Lord, have spoken and done it,' declares the Lord." (Ezekiel 37:3-6, 11-14)
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Feel free to share this email with your friend.
God bless you and your friend!
May more Jewish people believe in Jesus!