@Vidmastereon @Wormwood @ninja8tyu
If you want to go deeper as to why the Christian image is present you can look at the crucifixion of Japanese.

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A list of all the Christians in Miyado and Osaka was made out, and on 5 Feb., 1597, 26 Christians, among whom were 6 Fransciscan missionaries, were crucified at Nagasaki. Among the 20 native Christians there was one, a child of 13, and another of 12 years. "The astonishing fruit of the generous sacrifice of our 26 martyrs" (wrote a Jesuit missionary) "is that the Christians, recent converts and those of maturer faith, have been confirmed in the faith and hope of eternal salvation; they have firmly resolved to lay down their lives for the name of Christ. The very pagans who assisted at the martyrdom were struck at seeing the joy of the blessed ones as they suffered on their crosses and the courage with which they met death".

https://www.newadvent.org/cathen/09744a.htm
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The theme of sacrifice is present in the first episode of Ultraman. To the Christians that died martyrdom was the most heroic thing they could ever do. Eiji Tsuburaya is heavily implying this aspect in his stories as heroic.

(It's also probably really cool to him and something to use to make stories interesting. I can't image Christian imagery was common during postwar Japan, so it could be literally considered alien to people. )
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