Regarding the OP/thread topic, a few things to consider (IMO):
1) in the churchES, there exists both believers [saints] and those who are not [genuine saints]; whereas "the Church which is His body" consists ONLY of believers/saints [of 'this present age'... from the first century, up to our Rapture]
2) both "hot" and "cold" seem to be positives, from God's perspective (for it says, "I would thou wert hot or cold")
3) it is my understanding that "hot" and "cold" were waters originating from a particular source, whereas "lukewarm" was that which was not connected with such a source (sort of like a stagnant mudpuddle versus a river/stream [or aqueduct] or hot-springs kind of thing); thus, God "spews out of His mouth" the "lukewarm" who are not connected with the source: that is, He is looking to taste "Christ" in them, and lacking that, spews them out. So I tend to believe the "lukewarm" are those [in the churchES; i.e. in Christendom] who come in His name, but who are not vitally connected with/to The Source (Jesus Christ) [even Matt7 says some will say, "haven't WE DONE... many wonderful works" IN THY NAME, and He says unto them "I never knew you"]… IOW, I do not see the Rev3 passage as a "lose your salvation" context
[note: I tend to believe the "lukewarm," here, then are spewed into the tribulation period and there counseled to "buy of Me gold tried in the fire" 3:18, where also it says, "...that the shame of thy nakedness do not appear," which is very close to what is also stated in Revelation 16:15-16 (just before the 7th vial is poured), "... Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame. And he gathered them [see verse 14?] together into a place called in the Hebrew tongue Armageddon" (these [those who heed His Word] apparently having obtained, in/during the trib, that which was previously lacking, just as Revelation 3:18 had instructed...)]
That's kind of how I see it.
1) in the churchES, there exists both believers [saints] and those who are not [genuine saints]; whereas "the Church which is His body" consists ONLY of believers/saints [of 'this present age'... from the first century, up to our Rapture]
2) both "hot" and "cold" seem to be positives, from God's perspective (for it says, "I would thou wert hot or cold")
3) it is my understanding that "hot" and "cold" were waters originating from a particular source, whereas "lukewarm" was that which was not connected with such a source (sort of like a stagnant mudpuddle versus a river/stream [or aqueduct] or hot-springs kind of thing); thus, God "spews out of His mouth" the "lukewarm" who are not connected with the source: that is, He is looking to taste "Christ" in them, and lacking that, spews them out. So I tend to believe the "lukewarm" are those [in the churchES; i.e. in Christendom] who come in His name, but who are not vitally connected with/to The Source (Jesus Christ) [even Matt7 says some will say, "haven't WE DONE... many wonderful works" IN THY NAME, and He says unto them "I never knew you"]… IOW, I do not see the Rev3 passage as a "lose your salvation" context
[note: I tend to believe the "lukewarm," here, then are spewed into the tribulation period and there counseled to "buy of Me gold tried in the fire" 3:18, where also it says, "...that the shame of thy nakedness do not appear," which is very close to what is also stated in Revelation 16:15-16 (just before the 7th vial is poured), "... Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame. And he gathered them [see verse 14?] together into a place called in the Hebrew tongue Armageddon" (these [those who heed His Word] apparently having obtained, in/during the trib, that which was previously lacking, just as Revelation 3:18 had instructed...)]
That's kind of how I see it.
I have a friend who believes the Revelation 2-3 "churches" are [solely] within the [future] tribulation period, but I tend not to agree with that based on the following (which subject I often post about, here)... IOW, I hold to the distinction found in Revelation 1:19 showing "the things which thou hast seen, the things which ARE, and the things which SHALL BE HEREAFTER" ("hereafter" referring to that which follows after our Rapture, the rapture of "the Church which is His body" [to whom the "Rapture" solely pertains], though "churchES" will still exist in the trib years, as I see it [but "the Church which is His body" will not be present on the earth in the trib years])--
"the beginning of birth PANGS [PLURAL]" (Matt24:4-8; Mk13:5-8 [and DESCRIBED in Lk21:8-11, but not so named, there]) :
--is the equivalent to "the SEALS" of Rev 6 (and Rev1:1 states, "The Revelation of Jesus Christ, WHICH GOD GAVE UNTO HIM [Jesus], TO SHEW UNTO His servants [note 7:3 (144,000 "servants of our God," for example)] things which must come to pass [note 4:1 (the "future" aspect of the Book !)] IN QUICKNESS [NOUN]" [not things which will unfold over the past 2000 years or so, but "IN QUICKNESS [NOUN]" [same as in Lk18:8[chpt17-end] re: "avenge," etc])
--is also equivalent to Matt24:4/Mk13:5 "G5100 - tis - 'A CERTAIN ONE' " the equivalent of the INITIAL "birth PANG [SINGULAR]" which is the "FIRST SEAL" (the "whose coming/arrival/advent//presence/parousia" [2Th2:9a] of the "man of sin" IN HIS TIME [Dan9:27a(26)-"the prince THAT SHALL COME"..."FOR ONE WEEK [7yrs]"]), that is, at the START of the 7-yr trib
So, I believe Revelation 4-5 is a scene which follows our Rapture, is when Jesus "STANDS to JUDGE" [Isa3:13, Lam2:3-4 (similar to 2Th2:7b-8a), etc], and is at the START of the future 7-yr tribulation period (the "IN QUICKNESS [NOUN]" time period leading up to His Second Coming to the earth).
I believe the "lukewarm" are not saved [that is, although they come in His name, they are not vitally connected to the Source, Jesus]... that is, are not saved before the Rapture (but may possibly heed His "counsel" stated in 3:18 and come to faith following the Rapture, meaning, within the trib years... but keeping in mind that 2Th2:10-12 is also a reality during that same future time period).