The freedom of choice:
Everybody is free to have choice; but his choice will decide his outcome in the afterlife:
>> So
Jews - when they convert to the Islam - will not lose their belief in Moses:
but they will have the additional blessings and grace of believing in Jesus Christ and Prophet Mohammed, and will acquire the wisdom of the Quran in addition to their belief in the Ten Commandments.
>> Similarly,
Christians - when they convert to the Islam - will not lose their belief in Moses and Jesus, but will add the blessings and grace of their additional belief in Mohammed and the Glorious Quran.
This is because
the Quran confirms Moses and Jesus and the Torah and the Gospel revealed to them; and the Quran is the recent revelation from God Almighty and Most Gracious.
(O believers [in Mohammed and the Quran],
believe in God and His messenger [Jesus] and the scripture [: the Gospel] which He revealed to His messenger [Jesus],
and the scripture [: the Ten Commandments] which He had revealed [to Moses] aforetime.
Whoso disbelieves in God and His angels and His scriptures and [all] His messengers and the Last Day, has surely gone far, far astray.)
The above between brackets is the explanation of the Quran 4: 136
يَا أَيُّهَا الَّذِينَ آمَنُواْ آمِنُواْ بِاللّهِ وَرَسُولِهِ وَالْكِتَابِ الَّذِي نَزَّلَ عَلَى رَسُولِهِ وَالْكِتَابِ الَّذِيَ أَنزَلَ مِن قَبْلُ وَمَن يَكْفُرْ بِاللّهِ وَمَلاَئِكَتِهِ وَكُتُبِهِ وَرُسُلِهِ وَالْيَوْمِ الآخِرِ فَقَدْ ضَلَّ ضَلاَلاً بَعِيدًا
The above aya was revealed because some Jews converted and became Muslims, and said:
"Mohammed, we believe in your message and religion because you invite people to God alone according to the First Commandment; but we don't believe in Jesus: he was only an impostor and sorcerer"
Therefore the above aya was revealed and told them they had to believe in Jesus as had they to believe in Mohammed and to believe in the Gospel as should they have to believe in the Torah and the Quran.
Double rewarding for their belief in Jesus and Mohammed:
And then they would have double fold of their reward:
one fold for their belief in Jesus, and the other fold for their belief in Mohammed.
As in the aya 57: 28
يَا أَيُّهَا الَّذِينَ آمَنُوا اتَّقُوا اللَّهَ وَآمِنُوا بِرَسُولِهِ يُؤْتِكُمْ كِفْلَيْنِ مِن رَّحْمَتِهِ وَيَجْعَل لَّكُمْ نُورًا تَمْشُونَ بِهِ وَيَغْفِرْ لَكُمْ وَاللَّهُ غَفُورٌ رَّحِيمٌ
The explanation:
(O believers [in Mohammed],
ward off God['s punishment, concerning your claim that Jesus was a sorcerer], and believe in His messenger [Jesus, the son of Mary]
and then [if you believe in Jesus], [God] will give you two portions of His mercy [: in the world of souls, and on the Doomsday; that is your belief in both Jesus and Mohammed],
and appoint for you a light wherewith you walk among souls] and forgive you [your past sins];
for God is Most Forgiving, Most Merciful.)
