Simple.
Read the account of Stephen’s martyrdom where he dies in the Acts of the Apostles. Notice carefully what Stephen sees just before he dies.
Acts 7:52-56.
The book of Daniel and Ezekiel talk about the “Son of Man” coming on the clouds of heaven; a sort of mirror image of the Apostles seeing Jesus “going away” on a cloud at the ascension.
There is a way in which heaven and God’s kingdom is present, now, but invisible. Some see it beautifully before they die. There is also, I suppose, in Church Teaching, a definitive second coming where all who are on earth will see Jesus before being transformed. (eg: not necessarily dying, in the corruption sense. )
Some see Jesus as king to strengthen them before death, other’s will see him definitively at the end of time. But this image of the “Son of Man” on the clouds of heaven is Jesus universally being proclaimed the judge of the whole world by his father.
Jesus enters his kingdom in heaven by riding upon the angels (clouds/wind/thunder/fire.). That’s what Stephen saw before he died.