...members of the body of Christ...
How could we Christians who will be in the tribulation after the cross and after Pentecost (i.e. not OT) who have washed our "robes, and made them white in the blood of the Lamb" (Revelation 7:14) and have "the faith of Jesus" (Revelation 14:12) and are "in the Lord" (Revelation 14:13) not be in his body?
...the terrible Day of the Lord that is coming...
Note that no verse says or requires that the day of the Lord begin before the 2nd coming.
I believe the day of the Lord is the day we wait and watch for, and will rejoice in:
"Waiting for the coming of our Lord Jesus Christ: Who shall also confirm you unto the end, that ye may be blameless in the day of our Lord Jesus Christ" (1 Corinthians 1:7-8).
"Yourselves know perfectly that the day of the Lord so cometh as a thief in the night... But ye, brethren, are not in darkness, that that day should overtake you as a thief... let us not sleep, as do others; but let us watch" (1 Thessalonians 5:2, 4, 6).
"We are your rejoicing, even as ye also are ours in the day of the Lord Jesus" (2 Corinthians 1:14).
Paul said the day of the Lord will come as a thief in the night (1 Thessalonians 5:2) and the Lord said he will come as a thief (Matthew 24:43-44), and there's no 3rd coming of the Lord.
I believe the subject of 2 Peter 3:4-10 is the promise of the Lord's 2nd coming: "Where is the promise of his coming?... The Lord is not slack concerning his promise... the day of the Lord will come as a thief in the night" (2 Peter 3:4, 9, 10).
...will be worse than all the suffering the world has ever seen...
I believe Jesus said the tribulation will be cut short because we the elect will still be here: "For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. And except those days should be shortened, there should no flesh be saved: but for the elect's (eklektos) sake those days shall be shortened" (Matthew 24:21-22).
I believe we in the church are the elect:
"Elect (eklektos) according to the foreknowledge of God the Father, through sanctification of the Spirit, unto obedience and sprinkling of the blood of Jesus Christ" (1 Peter 1:2).
"Ye are a chosen (eklektos) generation, a royal priesthood, an holy nation, a peculiar people; that ye should shew forth the praises of him who hath called you out of darkness into his marvellous light" (1 Peter 2:9).
"Put on therefore, as the elect (eklektos) of God... longsuffering" (Colossians 3:12).
"Paul, a servant of God, and an apostle of Jesus Christ, according to the faith of God's elect (eklektos)" (Titus 1:1).
Note that at the 2nd coming some of us elect will still be "alive and remain" (1 Thessalonians 4:15) on the earth: "And then shall he send his angels, and shall gather together his elect (eklektos) from the four winds, from the uttermost part of the earth to the uttermost part of heaven" (Mark 13:27).
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