What does Jesus mean by saying that he is returning to his Father?
Jesus never said he was 'returning' or 'going back' to his Father.
There are no words in the original text of Jesus 'returning' or 'going back'. These additions advance an idea the text does not endorse or intend.
See John 16:5,28, 13:1,3, 20:17, 14:28 for these additions - obviously each translation will vary.
John 20:17 should read like this,
Jesus said to her, "Stop clinging to me, for I have not yet ascended
to the Father; but go to my brethren and say to them, 'I ascend to my
Father and your Father, and my God and your God.'"
One might presume, depending on the theology, that 'returning' is implied. That response indicates a willingness to read into the text that which is consistently missing in every text about Jesus' ascension.
The Father was never not with Jesus in spirit. Jesus mission, opposing and conquering the epitome of evil, required constant support from the Father. As a man, Jesus had no super-powers; he said often he could do or say nothing of himself. John 5:30, 19, 8:28. Also Matt 9:8, Acts 10:38, Acts 2:22, John 9:3
Do you not believe that I am in the Father, and the Father is in me? The words that I say to you I do not speak on my own initiative, but the Father abiding in me does His works. John 14:10
It must be said that as Jesus was without sin, the relationship with his Father was of a nature we can barely imagine. As Jesus said, I and the Father are one - the idea of 'one essence/substance' or similar idea is not a biblical one. As the verses below show, it is explained as being one in mind and purpose - and that we too, may be so 'at one' with God.
Holy Father, protect them by the power of your name, the name you gave me, so that they may be one as we are one. John 17:11
that they may all be one; even as You, Father, are in me and I in You, that they also may be in Us, v21
In fact these verses strongly refute such an idea as, 'one substance' - it is a human construct not biblically supported.
He offered to his disciples this same connection that enabled him to finish the course,
"I will ask the Father, and He will give you another Helper, that He may be with you forever; John 14:16
and then, to clarify what he meant, identifying himself and the Father...
John 14:23 "Anyone who loves me will obey my teaching. My Father will love them, and we will come to them and make our home with them.
There are several expressions about, 'coming from heaven' or, 'from the Father'.
We are not meant to take these literally, as if they are implying travelling from heaven or was in heaven to begin with.
For I have come down from heaven, not to do my own will, but the will of Him who sent me. John 6:38
What sort of 'eternal Son' has a differing will to his eternal Father - especially if they are of, 'one substance'?
This is the bread which comes down out of heaven, that anyone may eat of it and not die. John 6:50, (58)
The first man is from the earth, earthy; the second man is from heaven. 1 Cor 15:47
How could the mortal 'man' Jesus be 'in heaven' with the Father before his change to spirit life?
The baptism of John—was it from heaven or from men? Mk 11:30
John answered, "A man can receive nothing unless it has been given him from heaven. 3:27
Every good thing given and every perfect gift is from above, coming down from the Father James 1:17
As can be readily seen, it's easy to interpret the words, without seeing the overall context of these expressions.
Certainly Jesus is/was not OF this world. This world of corruption and sin is not the world Jesus is from - he is from God and holiness - heavenly - not worldly.
He said of the disciples, 'they are not of this world' John 17:16 Yet or course, they were born in sin on earth and were earthly - but now they are OF heaven because they are God's - just as Jesus was God's.
Anything OF God is heavenly, even if it's just 'bread', serving a holy purpose, just as Jesus and John did with their lives too.
Jesus - according to the text, didn't 'return' as he was never there to begin with. Only once exalted to the Father is he actually with the Father. Remember Jesus is still flesh and bones - he is not a spirit, he can only be in one place at a time! (Luke 24:39)
The Father was never not with Jesus in spirit - the Father sent Jesus the holy spirit, how can they be apart?
When Jesus departed to be with the Father, he was 'taken up', or 'carried', 'lifted' - not something that one does of oneself. Even here as Jesus being born of the spirit, made immortal, waited on the Father to lift him up! Mk 16:19, Acts 1:9, Luke 24:51 Just as he waited on the Father to raise him from the dead. Finally, he was able to sit by God's side - glorious, victorious, King of Kings.