In Hebrews 9:24-28 we find three appearings of Jesus Christ. We may call these the present, past, and future appearings of the Son of God.
The Three Appearings
His Present Appearing —intercession
"For Christ is not entered into the holy places made by hands, which are copies of the true, but into heaven itself, now to appear in the presence of God for us."(Heb 9:24)
The present appearing of Christ is not on earth but in heaven. We cannot see him now (1Pe 1:3-9) but he is manifest in heaven, in God's presence and at his throne, "to make intercession for us" as our great high priest (Heb 7:25, 4:13-16).
His Past Appearing —sacrifice
"Not that he should offer himself often, as the high priest enters the Most Holy Place every year with the blood of another, for then he would have to suffer often since the foundation of the world. But now, once at the end of the ages, he has appeared to put away sin by the sacrifice of himself."(Heb 9:25-26)
The past appearing of Christ began with his birth in Bethlehem and ended with his death on the cross, his resurrection from the dead, and his ascension into heaven. There are many witnesses to the Son of God becoming flesh. For example...
Wise men from the east recognised him as the King (Mtt 2:2)
Simeon recognised him as the Saviour (Lke 2:29-32
John recognised him as the Lamb of God (Jhn 1:29)
Peter recognised him as the Christ (Mtt 16:16)
Thomas recognised him as Lord and God (Jhn 20:28)
Paul recognised him in these words, "he loved me and gave himself for me"(Gal 2:20)
His Future Appearing —salvation
"And as it is appointed for men to die once, and after this the judgment, so Christ was offered to bear the sins of many. To those who eagerly wait for him he will appear a second time, apart from sin, for salvation ."(Heb 9:27-28)
The future appearing of Christ was promised by Jesus when he said, "I will come again to take you to myself, that where I am there you may be also"(Jhn 14:1-3). He will come in the clouds and every eye shall see him (Acts 1:10-11, Rev 1:7). While the rest of the world will be terrified by the fiery angels destroying the world, Christ's followers will be caught up to meet him in the air, and he will take them away to ever live with him in heaven (1Th 4:14-18, 2Th 1:5-10).
The Greek for 1 and 2 above is , phaino 5319, to be manifest. For 3 above the Greek is optomai 3700, to be seen.