The first missionaries to arrive were Jesuit priests in the 16th century.They boldly confronted idolatry and immorality and baptized 1000s in a few years. But when they interfered with politics they were driven out and their converts were martyred, forced to recant or driven underground. For 300 years Christianity was banned under penalty of death and the church suffered the most thorough persecution in history. Not having the Word of God and not knowing the gospel, hidden Christians became syncretistic.
Protestant missionaries arrived in the late 19 century. Many Japanese impressed by the industrial strength of the west and seeking a new world view became nominal Christians. The motive often was become a Christian for the good of Japan. One leader wrote at that time -"I love two Js and no third; one is Jesus and the other is Japan, I don't know which I love more, Jesus or Japan". With this attitude most apostasized after a few years and very few could withstand the nationalistic pressure from the military government to compromise key articles of the faith.
By the end of WWII most churches were venerating the Emperor and bowing to shrines. After the war complete religious freedom was granted and many again became Christians but large numbers backslid soon afterward. We meet them all the time. At present the church is growing very slowly. Total church attendance in Japan increased only 1.5% last year and only 0.22% of the population attend church. Sunday schools are dwindling with the leisure boom hitting the country. There are so many with a surface interest in Jesus, lonely needy people, but the issue seems still to be Japan or Jesus. No man can serve two masters. We need the Holy Spirit sent from heaven!