For Trinitarians, why is it important to posit Jesus's human nature continued existence post ascension?

We pray " In the name of the Father and of the Son and of the Holy Spirit " ( Note the preposition 'of ' prefixed to each Person of the Trinity ! ). We do not pray " In the name of the Father and of Jesus...." . To elaborate, Jesus is the name by which we human beings recognise the Son of God . If you have a doubt on whether the Son of God exists, you should first doubt if God the Father exists.

Here is what the Bible says about Jesus Christ's continued existence:

"Jesus Christ is the same yesterday, and to day, and for ever." (Hebrews 13:8)

That should settle it!

I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death. [Revelation 1:18 KJV]

We Christians believe that Jesus Christ is alive - now and for ever - because we hear his word and we believe it. And we are aware of His Presence within, in Holy Spirit, as the scriptures testify . . . .

Know ye not your own selves, how that Jesus Christ is in you, except ye be reprobates? [2 Corinthians 13:5 KJV]

And if it be the bodily resurrection of Christ that is disbelieved then there is ample evidence in the scripture . . . . .

Ten Bible Verses Concerning "Doubting the Resurrection" of Jesus Christ

The Model

Before I studied theology as a layman (I'm a software engineer by training), I used to have those questions too, even though I was raised a Christian. I'm going to answer from the Trinitarian perspective by presenting a model that has helped me tremendously

• to understand the inner life of the Trinity (from the limited revelation God has given us)
• the Trinity's relationship to the human Jesus
• the Trinity & Jesus's divine mission to the world

through the lens of Nicaea and Chalcedonian formulas and the Church Fathers's reflection:

1. Method: How do we know? We are guided by the human being Jesus and what he did and what he said, in the context of the previous revelation of God recorded in the OT, which Jesus fulfilled IN PERSON. By the ONE God incarnating in a human person, God stoops down to our level so we can KNOW and LOVE Jesus (and by extension, God) in a mode appropriate to our nature, a CREATED being. Some of the most important Biblical foundation: John 17:1-5 and John 17:21-23.

2. What's the relationship between the 3 "persons" of the Trinity? There are many models, but the most helpful to me is the time-tested Aquinas's subsistent relations model where:

   • the KNOWING of God the Father eternally generated the LOGOS, which in turn became the pattern in all creation as all creatures participated in God, from inanimate mountain to vegetation to animals to rational human beings to angels. The preceding is a theological elaboration of what Col 1:15-17 says.
   • the LOVING of God the Father and of the Son eternally spirated the HOLY SPIRIT, which becomes to source of all human loves which started corrupted because of the Fall, but if we are willing to cooperate, can become the source of grace so our loves can be made more and more like Christ's love.

3. Are there two beings (Jesus and the Logos)? No. Jesus is a CREATED human being OF GOD (like an avatar, but more than this), but this 100% genuine human being is in hypostatic union with the UNCREATED God, thus preserving the ONE being, starting with his conception in Virgin Mary and continues to all eternity. The key concept is how God the Word who has always existed (even "before" universe was created) and who by nature is Spirit (John 4:24) has assumed flesh (adding the created human nature of Christ, John 1:14) without losing any of His divine nature. Navigating through much misconceptions in the first 400 years, the church fathers settled on the Chalcedonian formula: one person, two natures. Thus Jesus operates in BOTH divine and human mode, sometimes divine only, sometimes human only, and sometimes both.

4. What's the relation between eternity (Trinitarian God) and time (Jesus)? Understood philosophically, Jesus's human nature and his acts in history is a temporal effect of God who exists for all eternity (both outside time AND encompasses all points of time in the temporal universe). If I remember correctly, Boethius got the credit for this explanation, which subsequent church fathers and theologians adopted.

5. Why Jesus had to die? The main purpose (besides the obvious one to become the eternal Passover Lamb whom God provided out of love to pay for our sins) is solidarity and compassion. This is so we have a HUMAN EXAMPLE on how to live out God's own love and compassion for our neighbors (Jesus lived what he preached and commanded). The lecture by Fr. Thomas Joseph White (Aquinas on the Incarnation: part 1, part 2) beautifully show how it is appropriate and fitting that we are saved and glorified through God's INCARNATION in Jesus.

Answering your specific questions

• Does Jesus still exist? Yes, the human nature of Jesus exists forever and Jesus now sits "at the right hand" of God the Father until he comes again to judge the quick and the dead (Apostle's creed).
• The Father was just active in the beginning. This is a misconception. Father, Son, and the Holy Spirit are all united in mind and will for all time. Don't confuse this with progressive revelation, where God the Father had the "starring role" in the OT, Jesus in the gospels, and the Holy Spirit in the church today. Every act of Jesus and the Holy Spirit as part of the divine mission in the world is the act of the whole Trinity (i.e. Father was never excluded).
• Everything he did and everything he was intended to do has been done. Another misconception. Since I was a child I was taught to sing "What a Friend we have in Jesus" and to pray to Jesus. Since I was a teenager I was taught that Jesus is present in the Lord's supper. And for Catholics, Jesus is especially present in the Eucharist. Then when I learned theology, the concept of being united in Jesus applies in this age as well (!!) because this is the basis for our receiving life from Jesus our Savior. Jesus told us that he is the bread of life and that we ask God for our daily bread, which means in our life on earth.
• ... How comparable is this then really to us? The right way to think this is that Jesus in his human nature is exactly like us (but without sin) and his CREATURELINESS is on par with us, so in this regard he IS comparable to us. But since Jesus's human nature was ADDED to his preexisting divinity, he IS ALSO MORE THAN US, possessing the divine grace that can sanctify us and can lift us out of corruption (death, proclivity to sin, selfishness, etc.). You have a good point though, that although by the Holy Spirit we are being made to conform more and more like the HUMAN Christ, even in our glorified state after we die, we will forever remain a CREATURE just like the glorified human being of Jesus Christ today.
• Why are we sure he still exists as a separate person? It's wrong to say Jesus is a "separate person": there is only one person and one being who operates in two natures. So we can rephrase your question as: "Why are we sure he still exists in the flesh?" Because the Bible says so: Jesus is the "Son of Man" is understood by Trinitarians to mean the human nature of Jesus. The Son of Man will come again, in his human nature (but who is also God). In a crude way, we can think Jesus as the "human extension of God", an added nature since the Virginal conception. It's a Nestorian heresy to think Jesus was born a human and THEN indwelt by God; Jesus was Logos FIRST, then assumed human nature.
• Why couldn't he have united with the Father as soon as his mission on earth was completed? Again, a misconception. Trinitarian theology holds that since conception Jesus's human nature is perfectly united with the Trinity, which is a model of a saved human being who ALSO has the Trinitarian life inside us. It's just Jesus since conception has a perfect unity while we are still a work in progress. Maybe the misunderstanding has to do with Jesus not only assuming a human nature, he was ALSO assuming a low status of being a servant ON PURPOSE (for solidarity with the weakest and the most downtrodden of us) but God lifted Jesus to glory after accomplishing his human mission on the cross.
• Why do we pray to Jesus Christ and not to God? In the Trinitarian and Chalcedonian understanding, there is only ONE being that manifests to us in THREE "person". Our prayer is then tailored to our different circumstances addressing different "person" appropriately. So when we are persecuted because of Jesus, it's natural to address our prayer to Jesus who suffered on the cross because of who he was. When we are aware that we need more grace to overcome our selfishness, we pray to the Holy Spirit to give us more love (as Holy Spirit is love). When we are thankful of the beautiful nature, we pray to God the Father the Creator. When we want to know the truth we pray to God the Son (the Logos). Etc.

For Further Study

First, I highly recommend these 2 interviews with a Trinitarian scholar Fr. Thomas J. White. These 2 interviews have been tremendously helpful for me to answer common questions modern people have on the Trinity and the Incarnation:

1. Thomas Joseph White on Tough Trinity Questions

2. Thomas Joseph White on Tough Christology Questions

Next I highly recommend listening to the following lectures from the Thomistic Institute (which I indexed here) which gives us practical consequences of the above model for our spiritual life:

1. Christ and the Sharing of Divine Communion in Cyril of Alexandria about how through adoption and through communion we are participating and sharing the Trinitarian life. We can do this because we are united to the glorified human Christ who not only still alive, but will be forever more. If the hope in 1 Cor 15 is true, we too, will have a glorified human body as well as reaching the next level of participation in the Trinitarian life, which includes the beatific vision.

2. Divine Indwelling and the Presence of God about how God is present to us in at least 3 ways, and how we can increase our awareness of his presence and allowing God to sanctify us (because the source of holiness, the Trinitarian life given to us, is IN our souls).

3. The Heart of Salvation: Christ and the Christian in the Macarian Writings about sanctification, as the Trinitarian life inside us helps us to battle our sinful nature.

4. The Cross as Revelation of the Divine Life in Gregory the Great about the intimate relationship between compassionating with others (modelled after Jesus) with better path to increased contemplation and potential for beatific vision on earth.