Christians are not addresses in the Book of Revelation after the third chapter.
Christians should never be addresses! ;-) Christians as a universal body are not addressed in the first 3 chapters, either. The letters are to 7 specific, historical churches that existed when John wrote the letter.
What 600,000? You mean the 144,000? They are following Jesus, BTW. Rev 14: 4. These are they which were not defiled with women; for they are virgins. These are they which follow the Lamb whithersoever he goeth. These were redeemed from among men, being the firstfruits unto God and to the Lamb
Christians are martyred throughout the tribulation, see Rev 12:17 17And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.