In his 1937 lecture “Religion und Naturwissenschaft,” Planck expressed the view that God is omnipresent and held that “the holiness of the unintelligible Godhead is conveyed by the holiness of intelligible symbols.” Atheists attach too much importance merely to the symbols. On the other hand, “understanding without symbols would be impossible.” Planck, who from 1920 until his death was a churchwarden at Berlin-Grunewald, professed his belief in an almighty, omniscient, beneficent God, although he did not personify him. The Godhead is “identical in character with the power of natural law.” Both science and religion, although starting from different standpoints, wage a “tireless battle against skepticism and dogmatism, against unbelief and superstition,” with the goal: “toward God!”
In that same lecture, 10 years before his death, he said:
"Under these conditions it is no wonder, that the movement of atheists, which declares religion to be just a deliberate illusion, invented by power-seeking priests, and which has for the pious belief in a higher Power nothing but words of mockery, eagerly makes use of progressive scientific knowledge and in a presumed unity with it, expands in an ever faster pace its disintegrating action on all nations of the earth and on all social levels. I do not need to explain in any more detail that after its victory not only all the most precious treasures of our culture would vanish, but — which is even worse — also any prospects at a better future."
Planck was a devout Christian his entire career. Why rely on rumors when his own words are publicly available.