The Portuguese brought Christianity to Japan in 1549 but 100 years later, when the Shogun started to fear that Christianity was a precursor to European imperialism, Japan began wiping out Christianity. They instituted a sort of Buddhist Inquisition, ferreting out Christians and making them renounce their faith by having them step on the cross or defile other Christian iconography. If they refused to renounce their faith, they were tortured or killed. This went on until 1637, when a rebellion of 37,000 Christians led by the most famous Christian in Japanese history, the samurai swordsman turned ronin, Amakusa Shiro, took on the Shogun's army at Harajo castle...
...The Tokugawa Shogunate had one Christian lord, daimyo Konishi Yukinaga. When Yukinaga was stripped of his title, his samurai became ronins and were exiled to become farmers in Southern Japan. The brutal daimyo Matsukura Shigeharu, who replaced Yukinaga, was ordered to wipe out Christianity. To make a long story short, during what was known as the Shimbara Rebellion, Shigeharu marched on these farmers with a vast army of samurai warriors and were met with great resistance by an army of peasants and ronins who were under the leadership of Jinbei's son Amakusa. Amakusa's battle cry was, "We would rather die one swift death than a thousand slow ones."
When asked how a samurai could become a Christian knowing full well that he could only serve one master the Shogun Shimahara says, "The priests were effective and exemplified many of the bushido traits of self-sacrifice and serving a master, their master being Christ. The Portuguese were passionate people able to make an arduous journey, and were focused and dedicated, traits that inspired the samurai.
Also, there is evidence that many of the Christian samurai class or above took advantage of the situation because of trade knowing if they were Christian they would have an in with traders from Spain, Portugal and other European countries, so it was a motivating factor. There were earnest Christians, it was a new frontier and a new kind of thinking."