When Jesus preached, He called people to enter the kingdom (Luke 13:24). Sometimes He urged people to be saved (John 5:34). And other time He spoke of inheriting eternal life (Mark 10:30).
All three expressions point to the reality that occurs at conversion. When a person trusts Christ, that person is saved, inherits eternal life, and enters into the kingdom of God. Believers come under God’s rule and authority, not physically in heaven, but in every spiritual sense the influence of Christ’s government extends to them.
So while we do not yet live physically in heaven, we do have our spiritual citizenship in the heavenly realm. Therefore we should be preoccupied with heavenly things. In fact, we are commanded to contemplate heaven; to pursue it the way Abraham sought the city of God, and fix our affections there.
This means earnestly purging worldliness from our hearts. It means learning to wean ourselves from the preoccupations of this life. It means looking ahead to eternity and living in the expectation of a sure and certain hope. It means looking away from the mundane and temporal, and fixing our eyes steadfastly on Him who is the glory and the centerpiece of heaven.
Those who live with this heavenly perspective discover abundant life as God intended it here on earth. Ironically, those who pursue earthly comforts are really the most uncomfortable people on earth. As the Puritan theologian Richard Baxter wrote,
A heavenly mind is a joyful mind; this is the nearest and truest way to live a life of comfort, and without this you must needs be uncomfortable. Can a man be at a fire and not be warm; or in the sunshine and not have light? Can your heart be in heaven, and not have comfort? [On the other hand,] what could make such frozen, uncomfortable Christians but living so far as they do from heaven? . . . O Christian get above. Believe it, that region is warmer than this below.  Richard Baxter, The Saints’ Everlasting Rest, abridged by John T. Wilkinson (1650; reprint, London: Epworth, 1962), 110.
Baxter went on to write,
There is no man so highly honoureth God, as he who hath his conversation in heaven; and without this we deeply dishonor Him. Is it not a disgrace to the father, when the children do feed on husks, and are clothed in rags, and accompany with none but beggars? Is it not so to our Father, when we who call ourselves His children, shall feed on earth, and the garb of our souls be but like that of the naked world, and when our hearts shall make this clay and dust their more familiar and frequent company, who should always stand in our Father’s presence, and be taken up in His own attendance? Sure, it beseems not the spouse of Christ to live among His scullions and slaves, when they may have daily admittance into His presence-chamber; He holds for the sceptre, if they will but enter.  The Saints’ Everlasting Rest, 118.
Unfamiliarity with heaven makes a dull and worldly Christian. God has graciously bid us sample the delights of the world to come, and it is only a rebellious and perverse mind-set that keeps us mired in the mundane and worldly. God has given us a down payment on heaven. He has transferred our citizenship there. We “are no longer strangers and aliens, but . . . are fellow citizens with the saints, and are of God’s household” (Ephesians 2:19). We therefore cannot ignore heaven’s glory as if it had no significance. In Baxter’s words, “There is nothing else that is worth setting our hearts on.”  The Saints’ Everlasting Rest, 121.
I know few truths in Scripture that are more liberating to the soul than this:
Our citizenship is in heaven, from which also we eagerly wait for a Savior, the Lord Jesus Christ; who will transform the body of our humble state into conformity with the body of His glory, by the exertion of the power that He has even to subject all things to Himself. (Philippians 3:20-21)
That is where our hearts should be. The cares of this world are nothing but a snare and a deadly pit. Jesus characterized “the cares of the world and the deceitfulness of riches and the desires for other things” as unholy diversions that “enter in and choke the word” (Mark 4:19). Similarly, the apostle John writes, “All that is in the world, the lust of the flesh and the lust of the eyes and the boastful pride of life, is not from the Father, but is from the world” (1 John 2:16).
“But we have the mind of Christ” (1 Corinthians 2:16). We can fix our hearts on the eternal glory of heaven, not on the things of this world, which inevitably come to naught anyway (1 John 2:17). We are members of a new family, having become the children of God (John 1:12). Galatians 4:26 says that “the Jerusalem above is free; she is our mother.” We have a new citizenship (Philippians 3:20), new affections (Colossians 3:2 KJV), and a new storehouse where we are to deposit our treasures (Matthew 6:19-20).
Best of all, we can live in the glow of heaven’s glory here and now, with our hearts already in heaven. This is to say that the Christian life is meant to be a foretaste of heaven on earth. Believers can daily partake of the sweet, satisfying benefits of the same heaven to which someday we will go to dwell forever. Praising and loving God with all our being, adoring and obeying Christ, pursuing holiness, cherishing fellowship with other saints—those are just some of the elements of heavenly life believers already savor in this world. Those same pursuits and privileges will occupy us forever, but as we see the fruit of the Holy Spirit come to maturity in our lives, we should begin to enjoy and treasure the goodness of heaven in a very full sense even now.