Adam did not "image" God by portraying some aspect of his appearance. Adam and Eve (collectively) were created in the image and likeness of God, to rule, relate, and procreate.
The characteristics of God's personality which are reflected in mankind are therefore immaterial: His rulership, relational nature, and creative power. All of these are inherent in the "image" and "likeness" when they were created:
Then God said, “Let Us make man in Our image, according to Our
likeness; let them have dominion over the fish of the sea, over the
birds of the air, and over the cattle, over all the earth and over
every creeping thing that creeps on the earth.” So God created man in
His own image; in the image of God He created him; male and female He
created them. Then God blessed them, and God said to them, “Be
fruitful and multiply; fill the earth and subdue it; have dominion
over the fish of the sea, over the birds of the air, and over every
living thing that moves on the earth.” — Genesis 1:26-28 NKJV
- rulership: mankind is given dominion over the earth
- relationship: male and female reflect the plurality (fellowship) in "Let us.." and "our image"
- procreative: they subdue the earth by multiplying (in a similar way that God is reproducing his image)
A useful analogy here is that of a portrait. Suppose I showed some friends of yours an old portrait of you. They would recognize immediately that it was you, but none would mistakenly believe that the physical potrait WAS YOU. It is an image of you, and so we say "it is you," knowing it is only an image of you.
This is why it can be said of Jesus in Colossians 1:5 "He is the image of the invisible God." God is invisible, but Jesus is able to "image" him by perfectly demonstrating his immaterial nature in material form.
Jesus took this form on, but it was not so from the beginning.
who, being in the form of God, did not consider it robbery to be equal
with God, but made Himself of no reputation, taking the form of a
bondservant, and coming in the likeness of men. — Philippians 2:6,7 NKJV
And the Word became flesh and dwelt among us, and we beheld His glory,
the glory as of the only begotten of the Father, full of grace and
truth. — John 1:14 NKJV
Inasmuch then as the children have partaken of flesh and blood, He
Himself likewise shared in the same, that through death He might
destroy him who had the power of death, that is, the devil, — Hebrews
Jesus, who shared his divine nature with the Father, became a human being (he was begotten of the Father), but it was to share in our physical nature so that we can share in his divine nature.
The meaning of "spirit" holds no mystery here. It means a person that is immaterial.
You can see this whole theme throughout the book of John. Jesus talks about spiritual things and his hearers confuse them for physical or earthly things:
- The Jewish leaders say "it took forty-six years to build this temple" when Jesus says he will destroy the temple. He understands that he is the actual dwelling place of God, not the physical temple.
- Nicodemus asks "must I go into my mother's womb a second time?" when Jesus says he must be born again. Jesus is talking about spiritual birth.
- The woman at the well says "give me this water that I won't have come and draw any more" when Jesus says he will give her the living water, but Jesus is talking about the Holy Spirit.
- Many disciples leave, saying "this is a hard saying" when Jesus says they must eat his body and drink his flesh. Jesus clarifies that he is talking spiritually, not physically.
It is in the midst of this thread that Jesus says: "God is Spirit, and those who worship Him must worship in spirit and truth." So it should be clear that "worship in spirit" is being contrasted with physical modes of worship such as the temple.
Jesus is divine because he is a spirit, not because he is a human being. This does not deny that he became a human being. But he did so to make us like him in spirit. The destiny of human beings is to be united with God in his spiritual nature. This is why he says:
Nevertheless I tell you the truth. It is to your advantage that I go
away; for if I do not go away, the Helper will not come to you; but if
I depart, I will send Him to you. — John 16:7 NKJV
It is to our advantage because now God is indwelling us in the Holy Spirit. This is better than being localized in a human body.
Additionally, we can exist without bodies and still be ourselves. So we might say we are spiritual beings live in bodies. Our human bodies are not essential to our nature.
So we are always confident, knowing that while we are at home in the
body we are absent from the Lord. For we walk by faith, not by sight.
We are confident, yes, well pleased rather to be absent from the body
and to be present with the Lord. — II Corninthians 5:6-8 NKJV
To answer your question, can it be taken to mean "God is only a spirit" the answer is yes. God has and always be a Spirit. That is his essential nature. That fact that a spirit may inhabit a body does not change this.