One the KEYS for understanding the Sign of the Son of Man is that it is a Progressive Sign for ALL to SEE! Notice Jesus tells what to look for as the answer to the question. “Tell us, when will these things be, and what will be the sign of your presence and of the conclusion of the system of things?” (Matthew 24:3)
Since Jesus is not returning to the earth as a man, his Parousia would require a Sign for ALL to SEE and WITNESS, whether they understood what they were seeing or not. Brother Russell wrote: "'The Earth Abideth Forever' Christ will not return to Earth as a Man - Present Year Believed to Mark Great Change of Dispensation, but World May not discern Immediate Difference." The Bible Students Monthly, January, 1914.
Since Christ will not return to Earth as a Man, Brother Russell wrote: "Our Lord, in describing His Second Presence, clearly indicated that it would be unnoticed by the world until the cataclysm of trouble should come. Thus He likened the earlier days of His presence to that period before the Flood, when the world, unconscious of the impending catastrophe, continued to build, eat and drink as usual, and knew not. So, says the Master, it will be in the end of this Age. Eating, drinking, building, planting and marrying will proceed as usual; and the world will not know that they are in the PRESENCE of the Son of Man."
Notice he didn't write Second Coming, but rather 'Second Presence'? Brother Russell was speaking of Christ's PAROUSIA--PRESENCE. He continues, "The Bible nowhere tells that Jesus took the human nature to keep it forever, and to return with it to Heaven, where it would be completely out of order and out of place. The Bible teaches, on the contrary, that "flesh and blood cannot inherit the Kingdom of God" (1 Cor 15:50); and that Jesus was made flesh merely 'that He, by the grace of God, should taste death for every man' (Heb 2:9) and that after doing this work He would "ascend up where He was before." (John 6:62) The great Day of Trouble, noted throughout the Scriptures as the Time of Trouble such as never was since there was a nation, will come upon them suddenly."
Brother Russell also mentions that this time of trouble could last a certain amount of time as in the time of Noah. Then he mentions, "Then, later on, will come the Epiphania; that is to say, the revelation, or manifestation, of the present One. This will not be a manifestation in the flesh, but in a great Time of Trouble, symbolically represented as fire, as when we read, "He shall be revealed in flaming fire, taking vengeance." -- 2 Thessalonians 1:7-10.
Christ Jesus referred to this sign when he said: “Then the sign of the Son of man will appear in heaven, and then all the tribes of the earth will beat themselves in lamentation, and they will see the Son of man coming on the clouds of heaven with power and great glory.” (Matt. 24:30) Thus the 'age of violence' was ushered in as the whole world experiences the Last Days. This age or time period will indeed come to an end. (Matthew 24:14)
It's Spiritual. Jesus explains in detail what the Sign of the Son of Man is...
Also, there will be signs in the sun and moon and stars, and on the earth anguish of nations not knowing the way out because of the roaring of the sea and its agitation.
People will become faint out of fear and expectation of the things coming upon the inhabited earth, for the powers of the heavens will be shaken.
And then they will see the Son of man coming in a cloud with power and great glory.
But as these things start to occur, stand up straight and lift up your heads, because your deliverance is getting near.
There is even more detail when we add the information from the vision in the Book of Revealtion that the Sign of the Son of Man as been unfolding ever since 1914!
Based on Brother Russells' study of the timing of the riding of the four horsemen in 1876, he submitted an article for publishing in The Bible Examiner, which had pointed forward to 1914 as the date for “the times of the Gentiles” to end, with serious consequences for the whole world of mankind.
Then the sign of the Son of man will appear in heaven, and then all the tribes of the earth will beat themselves in lamentation, and they will see the Son of man coming on the clouds of heaven with power and great glory. (Matt. 24:30)
Look! He is coming with the clouds, and every eye will SEE him, and those who pierced him; and all the tribes of the earth will beat themselves in grief because of him. Yes, Amen. (Rev 1:7) This prophecy includes the anointed would also be affected.
Better times lay ahead, but not for everybody! Since the Sign of the Son of Man is a Progressive Prophecy manifesting since 1914 for ALL to SEE, all the tribes of the earth are now lamenting in grief. Second Peter Chapter three, describes in detail, the timing of Jehovah's Day. It is a short transition period that will bring an end to this 'age of violence' then quietly ushers in the Great Crowd whom are personally invited by Jesus Christ with the marking of suvivorship. Soon, Satan's Dying World will be forced to see this revealing phenomenon of the Sign of the Son of Man and it will prove too much for Satan's Entire Political System of things! Therefore, it is said: “Awake, O sleeper, and arise from the dead, and the Christ will shine upon you.” (Ephesians 5:14)
“But on the day that Lot came out of Sodom it rained fire and sulphur from heaven and destroyed them all. The same way it will be on that day when the Son of man is to be revealed.” (Luke 17:29, 30)