In his commentary on Paul’s letter to the Philippians, D. Martin Lloyd-Jones wrote:
“We have a new citizenship, we are in Christ, and because we are in Christ, we are seated in the heavenly places with him. Certain things will happen to us before we finally arrive in heaven, but our citizenship is as definite now as it will be then. There will be a physical translation when we die, but spiritually we are there already, we belong there.”

What, then, does this citizenship mean? Why does Paul use this term to describe the Christian? I think it is to illuminate a contrast. The contrast between the Christian and the non-Christian.
Because, you see, when we look with the eyes of flesh to compare ourselves as Christians with those who are not, there is only the ability to see the fleshly differences. So beyond physical differences, we might see some behavioral differences pertaining to our church habits.

But the Christian lives in a spiritual realm...has his citizenship there, and in that realm there is infinite difference between its citizens and the citizens of this world. Until the Christian sees those differences, he has no power or incentive to behave ‘other-worldly’.

The Bible explains the vast differences between the Believer in Christ, and the unbeliever.
First of all, there are only two kingdoms, and everyone belongs to one.
Either you are a citizen of the Kingdom of heaven, or you are a citizen of the kingdom of this world.
The Kingdom of God, or the kingdom of Satan.
The Kingdom of Light, or the kingdom of darkness.
The Bible talks about the prince of the power of the air (Eph 2:2), who controls the lives of those who are not believers, and Paul contrasts this picture with terms that relate to the believer; such as. “Heirs of God and fellow heirs with Jesus Christ” (Romans 8:17) and “fellow citizens with the saints and ... of God’s household” (Eph 2:19)
The point that I want to make clear to you here, is that these terms are all in the present tense.

It is not a future thing that we look forward to ~ it is not a state or status that we hope to attain to ~ it is not an honorary title like might be given to a well-loved celebrity in his old age by a prestigious university or fraternity.

I recently saw a news blurb, where a famous American was bestowed Knighthood by the Queen of England. They made clear on the newscast that as an American, he cannot now call himself, “Sir -so and so”, as can a British subject like actor/director, Richard Attenborough, for example. He can put KBE at the end of his name, standing for “Knight of the British Empire”; but the title is honorary, and does not endow him with rights as a British citizen.
This is NOT the type of citizenship we’ve been granted in Heaven.

As believers in Christ we are raised up with Christ (Col 3:1), we have been delivered from the domain of darkness, and transferred to the kingdom of His beloved