The answer is, the Holy Spirit, the One who indwells all born again believers (Jn. 14:17), "plants that thought." Jesus said that when we are indwelt by the Spirit, He will live up to His name: He is the Comforter! John, in his Gospel, calls the Holy Spirit the "Comforter" four times (Jn. 14:16, 26; 15:26; 16:7). And, because John wrote his Gospel many years after Christ's Ascension, he knew from experience the work of the Holy Spirit in the hearts and minds of suffering believers. He obviously had full knowledge of the fate of each of the other Apostles, who had all been martyred for the preaching of the Gospel. And he, himself, while exiled on the Isle of Patmos (Rev. 1:9), authored the Book of Revelation, an entire book filled with tribulation, despair, but ultimately, victory!
However, in the mean time (the word "mean" seems so appropriate here), because of the curse of God on our planet and mankind in Genesis 3:16-19, until the victory of God in the Book of Revelation, man will experience hardship, including disease, famine, pestilence, war, and death. And, contrary to the "name it and claim it," "positive thinking," "prosperity gospel" "wolves in sheep's clothing" (Mt. 7:15), God's children will suffer, as well (Jn. 16:33). Yes, we will have tribulation, but nothing compared to that experienced by the lost during THE Tribulation (Dan. 9:27; Mt. 24:3-28; Rev. 6:1-18:24)!
But there is good news! One day, approximately 1007 years from now, after the seven-year Tribulation (Dan. 9:27), and the 1000 year Millennium (Rev. 20:1-7), that curse will be lifted! John wrote in Revelation 22:3 says, "And there shall be no more curse: but the throne of God and of the Lamb shall be in it; and His servants shall serve Him."
Believers are still in this world to spread the good news: WE WIN!