Rev 7:9-17

ELECT FROM THE GREAT TRIBULATION
John’s vision of the redeemed, now turns from the Sanctified Church, the 144,000, to the elect of the Great Tribulation.

These are the saints of the past who could not obtain the assurance of salvation from the Spirit of God, but “they overcame by the blood of the lamb and the word of their testimony” (Rev 12:11) … “they washed their robes and made them white in the blood of the Lamb” (Rev 7:14).
9 After these things I saw, and behold, a great multitude, which no man could number, out of every nation and of all tribes, peoples, and languages, standing before the throne and before the Lamb, dressed in white robes, with palm branches in their hands. 10 They cried with a loud voice, saying, “Salvation be to our God, who sits on the throne, and to the Lamb.” 11 All the angels were standing around the throne, the elders, and the four living creatures; and they fell before the throne on their faces, and worshiped God, 12 saying, “Amen! Blessing, glory, wisdom, thanksgiving, honor, power, and might, be to our God forever and ever! Amen.”
Rev 7:9
a great multitude – the number was great. Why is this number great, or uncountable, versus the 144,000? It seems logical to think that the elect in the Sanctified Church are the chosen few, who receive the holy spirit, whereas, the elect of the Great Tribulation period were the great number who were justified by faith, and overcame by the word of their testimony.

which no one could count – the meaning here cannot simply be that no man was able to count such a great number. Rather, it is a contrast to those in the Sanctified Church who can be “counted.” For example, we saw 12 men counted in Acts 19:7, “were about 12 in all.” This is what distinguishes the Sanctified Church – the visible and audible evidence of being sealed by the holy spirit. However, those who were justified by faith, in the Great Tribulation, are known only to God.

palm branches – the palm branches that compare the Spirit of Christ, as “the Body of Christ,” entering New Jerusalem (Heaven) are compared to the palm branches that welcomed Christ, as “the son of man” to Jerusalem, in Matt 21:8.
13 One of the elders answered, saying to me, “These who are arrayed in white robes, who are they, and where did they come from?” 14 I told him, “My lord, you know.” He said to me, “These are those who came out of the great oppression. They washed their robes, and made them white in the Lamb’s blood.
Rev 7:13
who are these arrayed in white robes? – these are the saints from the Great Tribulation, that began when the spirit of truth was taken from the Church, and the saints no longer had the testimony of the holy spirit, they no longer had the “seal of God.” Then Satan accused them before God “day and night” (Rev 12:10).

Rev 7:14
they have washed their robes – in Rev 12:11, we are told that those in the Great Tribulation overcame “because of the blood of the Lamb.” They obtained salvation, even though they were not sanctified by the spirit of God.
15 Therefore are they before the throne of God, they serve him day and night in his temple. He who sits on the throne will spread his tent over them.
Rev 7:15
His temple…tabernacle – the entry to the heavenly temple and tabernacle is also described in Rev 11:19, and Rev 21:3.

They serve Him day and night – these are the saints from the Great Tribulation who were previously accused before God “day and night” (Rev 12:10). Now they serve Him day and night.
16 They will never be hungry, neither thirsty any more; neither will the sun beat on them, nor any heat; 
hunger no longer, nor thirst anymore – from Matt 5:6, “blessed are those who hunger and thirst for righteousness, for they shall be satisfied.”

nor will the sun beat down on them, nor any heat – from Matt 20:12, those from the Great Tribulation are the labourers who bore the heat of the day, and received the same reward as those who were saved in the last days.
17 for the Lamb who is in the midst of the throne will be their shepherd, and will guide them to living springs of waters. God will wipe away every tear from their eyes.”
Rev 7:17
God will wipe away every tear – see also Rev 21:4; this is from Isa 25:8.