Sokei-an found, after long consideration, that the common basis of both Buddhism (the religion of wisdom) and Christianity (the religion of love) is non-ego.
"Love that does not come from Emptiness is merely partiality."
"Christian love, like the wisdom of Buddhism, is also non-ego."
"When we take this [non-ego] into ourselves, it is wisdom. When we give it to another, it is love."
"With this one key in my mind, Christianity and Buddhism come to know one another."