Complex sentence: Gromyko's neighbour at the time, Mikhail Sjeljutov, was a freethinker and introduced Gromyko to new non-religious ideas and told Gromyko that scientists were beginning to doubt the existence of God.
Split into simpler sentences: Gromyko's neighbour at the time, Mikhail Sjeljutov, was a freethinker and introduced Gromyko to new non-religious ideas. Sjeljutov told Gromyko that scientists were beginning to doubt the existence of God.