From that time Jesus began to preach, and to say, Repent: for the kingdom of heaven is at hand. Mat 4:17 The first and last message that Jesus preached to israel was one of repentance.      Understanding the change; being willing to change and actually changing is repentance.      The Kingdom Of Heaven is the Lord the Holy Spirit who issues from the Spirit Realm into the Physical Realm.

Neither shall they say, Lo here!     or, lo there!     for, behold, the kingdom of heaven is within you. Luk 17:21 Holy Spirit baptism establishes the Kingdom Of Heaven in a person.     This is so, because the Holy Spirit is the Spirit of King Jesus.      The Kingdom Of Heaven is not so much a place as it is a state of existence.      We are born into the Kingdom Of Heaven as New Spirit beings by Grace Through Faith in Christ Jesus.     Doing so, makes us eligible for Spirit Baptism.      Heaven is the Spirit Realm.     We enter into the Spirit Realm in the New Birth.      Not understanding that relationship with Holy Spirit - being in Him and Him in us - is major doctrinal error explaining the weakness of many believers.      Many believers go about searching in groups or churches or through gurus to experience the Kingdom Of Heaven not realizing that it is in them and must be experienced and manifested by faith.

We are Spirit Soul And Body, meaning we are spirit beings who have a soul and a body.   
The fact that we are spirit beings escapes most people because our default perspective is Carnal Minded.   

We know that the physical universe exists in 4 dimensions: length, width, height and time.   

Heaven is a set of dimensions (how many we can only guess) that we are unable to understand.   The Lord Jesus Christ came to inform us of heaven and to enable us to enter into heaven.   

And no man hath ascended up to heaven, but he that came down from heaven, even the Son Of Man who is in heaven. Joh 3:13 Jesus is prophesying His upcoming ascension on the basis that He came from heaven and is right now in heaven.      Heaven is the Spirit Realm.     Jesus walked in the Holy Spirit and so simultaneously existed in human flesh and in the Spirit.      In the New Birth we come into that dual existence.      We are privileged to and expected to walk in Newness Of Life

Jesus understood that He was in heaven at the same time He was on earth.   Let's think about what He said in Joh 3:13 this way: 'And no man has gone from the Physical Realm to the Spirit Realm, but he that came from the Spirit Realm into the Physical Realm, even the Son Of Man who is in the Spirit Realm and the PhysicalRealm.'

When Adam sinned, man's spirit was disconnected from access to the Spirit Realm, becoming Sin Nature which is totally Carnal Minded.   

When we are Born Again we become New Spirit beings who exist in heaven Joh 3:6 - i.e.   in the Spirit Realm.   

And hath raised us up together, and made us sit together in heavenly places in Christ Jesus: Eph 2:6 The New Spirit that we are In Christ Joh 3:5 2Co 5:17 is sitting in heaven in Christ in the Holy Spirit.     We are now One with Him in the Spirit Realm (heaven) Joh 3:6.      But to a large extent this new status is just potential as far as the Physical Realm is concerned.      The challenge for each believer is to enable the New Spirit being to manifest in our thoughts, speech and deeds.      This requires an act of our will for our minds to be transformed to the Mind Of Christ Phi 2:5.      To the extent that we allow the Word and Holy Spirit to renew our minds is the extent to which we manifest as Sons Of The Holy Spirit 1Jn 3:2 operating in the fruit and gifts of the Holy Spirit and so experiencing Abundant Life Joh 10:10.

So, we should not think of heaven as a different place but a different state of existence.   

If we live in the Holy Spirit, let us also walk in the Holy Spirit. Gal 5:25 The Holy Spirit is the Spirit Of Christ Jesus.     When we are In Christ, we are in His Holy Spirit and His Holy Spirit is in us.      Since we have New Life by the New Spirit it only makes sense for us to walk in the Spirit.     Walking implies that our physical lives are different - that we talk differently, that we do differently.      Jesus is fully man and He is fully God because His Spirit is the Holy Spirit.     He was not born with the Sin Nature that the rest of us were.     But He had to learn submission to the Father.     When He was baptised with the Holy Spirit He was empowered to walk in the Spirit.     He went to the wilderness fasting and praying until He was totally conformed to the will of the Father.     When He left the wilderness He was prepared to walk in the Spirit.      To walk in the Spirit is to believe and appropriate the promises of the Lord while fellowshipping with Him in the Holy Spirit.

We are not 'going to heaven'.   We are in heaven.   When Jesus comes back He will not take us to heaven.   He will institute the Kingdom Of Heaven on earth.   

The Spirit Realm has always existed because the Spirit and Soul of our Creator has always existed.   (He must exist somewhere).

In the beginning the Lord created the heaven and the earth. Gen 1:1 The Bible begins with the only beginning that man's mind can handle - Creator.     And our Creator created everything that is.      If a person is unwilling to believe this verse, they will be unable to believe anything that follows.      Out Lord is Creator of all things and the upholder of all things by the Word of His Power.     Heb 1:3

The heavens that were created are the parts of the physical universe excluding the earth.   Gen 1:1 speaks to the creation of the Physical Realm.   

The 'heavens' of the Physical Realm are above, or higher, than the earth.   
   → sky
   → planets
   → constellations