In Plato's work Phaedo, he introduces the word "triad" (in Greek τριάς),[105] which they[who?] translate as "trinity". This was adopted by 3rd and 4th century professed Christians as roughly corresponding to "Father, Word, and Spirit (Soul)".[106] Nontrinitarian Christians contend that such notions and adoptions make the Trinity doctrine extra-biblical.[citation needed] They[who?] say there is a widely acknowledged synthesis of Christianity with Platonic philosophy evident in trinitarian formulas appearing by the end of the 3rd century. They[who?] allege that beginning with the Constantinian period, these pagan ideas were forcibly imposed on the churches as Catholic doctrine.