Convincing another person to subscribe to your own beliefs or opinions on a particular subject has to be one of the most -if not the most effective way of validating your own reasoning. People who have confidence in their opinions or beliefs don’t necessarily need to convince anyone else of truthfulness or value of their convictions. On the other hand people who may not be 100% sure they got it right feel a lot better when they hear those magic words, you’re right!
There are two reasons why Jews were never big on selling Judaism to Gentiles:
We don’t believe that we have a monopoly on either truth or God. Every path to God and knowledge that inspires greater moral and ethical behavior is holy and has merits.
We frankly just don’t need anyone to validate our faith, heritage, beliefs, culture and values.
The early followers of Jesus Christ were a small, marginal group within the Jewish community. Not only did the majority of Jews at the time reject the idea that Jesus was the foretold messiah, but in time they became leery of and tried to exclude devotees of Jesus from the community.
To keep their young faith alive and flourishing, the followers of the Jewish preacher from Galilee marketed their new religion by promoting some enticing theologies:
Jewish scripture had suggested that the (human!) redeemer or messiah would bring everlasting peace and harmony to a troubled world. Jesus died having accomplished little by way of perfecting the human condition. Oops. How to fix that? The first Christians announced that Jesus was not simply the foretold messiah, but the human incarnate of that God that Jews had been worshipping since Abraham. And God’s don’t die do they? Ergo, Jesus’ death was just a bit of theater with a purpose: God gave the world His son so he could experience the drama and trauma of a human execution. Why? So Jesus could bear all our sins in the process of cleansing our souls for the afterlife. “He died on the cross so you could live. Believe in him and have eternal life.” A pretty nifty way of re-packaging the death of itinerant, charismatic preacher, no?
For those Jews and Gentiles who found studying and adhering to the 613 mitzvot/commandments of Judaism something of an inconvenience, the followers of Jesus made the Jews an offer they hoped many couldn’t refuse: Accept Jesus as your savior and you don’t have to bother with those obnoxious dietary restrictions or all those inconvenient, time-consuming rituals of Jewish life. Belief in Jesus nullifies the rest. Better yet, believe in Jesus and you will have eternal life.
Once you’ve accepted Jesus and the deed to eternal life has you name on it, you can take possession of it by selling the said formula for eternal life to non-believers.