Jesus Christ is the promised Messiah. He didn't came to make a new religion or to change something. Christianity is not a religion, but a Church. Among the Jews who accepted Jesus Christ there still were some who believed that the gentiles must obey the old regulations and laws in order to enter into the Covenant with God (the Jews considered themselves superior to other nations precisely because God made a Covenant with them alone). But Saint Paul was very clear when he said that in Christ there is no jew, no gentile, no man, no woman etc. So, in order to enter in this Renewed Covenant with God you must believe in Jesus Christ, confess one faith, one baptism for the remission of the sin, give up paganism (for gentiles) etc.
The conversion from judaism to christianity is not the conversion from a religion to another, but the ending of religion and beginning of the church.
Until today, there are people who don't understand this, who can't understand freedom in Holy Spirit which is characteristic for the Christian Church and they keep on blaming us for breaking the Sabbath, for example. In vain you explain to them that Jesus Christ resurrected Sunday morning thus bringing hope and joy to the world. They don't understand. They are afraid of joy and freedom.
I haven't read the Old Testament d'a capo al fine. I think that you can't read the Old Testament if you don't have an additional book to explain the difficult parts. I plan to study the judaism in the Old Testament as well as judaism today. I have a protestant friend (I am eastern orthodox christian) who is very cultivated and intelligent and he helps me with answers and he recommends me books. That being said, I'm a lazy person and would like that everything fall into my mouth without me having to do any effort.
No. I explained above why. Because the New Covenant is for everybody, Jesus Christ came on earth for all the world; and asking people to get circumcised, eat kosher and keep the Sabbath would be like not trusting in the saving sacrifice of Jesus Christ and in His resurrection. What Jews had with God (the Sabbath, the Covenant, the circumcision) was between them alone and God. Or, Jesus Christ is the Savior of the entire world, not of a nation alone.