Nonphysical beings and the nonphysical dimension to which they belong cannot be detected by physical instruments and are invisible to our physical eyes. Physical things have temporary existence because of physical laws. We can only conclude that the soul and spirit, being invisible and nonphysical, must be non-temporal, i.e., eternal.
The fact that our bodies are visible and thus temporal, but our souls and spirits are invisible and thus eternal, carries serious consequences. When comparing the short life expectancy on earth to eternity, the only rational choice one can make is to be far more diligent in preparing for the latter than for the former. Therefore, the Bible urges us to give priority to eternity over time. Jesus put it like this:
Lay not up for yourselves treasures upon earth, where moth and rust doth corrupt, and where thieves break through and steal: but lay up for yourselves treasures in heaven, where neither moth nor rust doth corrupt, and where thieves do not break through and steal. (Matthew:6:19-20)
The temporal treasures of earth are composed of visible things; the eternal treasures in heaven, like the soul and spirit that make up our true being, are not visible to our physical eyes. Treasures we lay up on this earth must all be left behind when we leave earth for what lies beyond death’s door. The great hope of the Christian is not to amass wealth in this life but to receive an eternal reward in heaven.
Peter described that reward as an “inheritance incorruptible, and undefiled, and that fadeth not away, reserved in heaven for you, who are kept by the power of God through faith unto salvation ready to be revealed in the last time” (1 Peter:1:4-5). In the resurrection, these bodies that decay in death are exchanged for new “spiritual bodies” (1 Corinthians:15:44), which are both visible and invisible, physical and spiritual, “eternal in the heavens” (2 Corinthians:5:1).
Paul encouraged Christians to believe in the midst of persecution and martyrdom, that
…our light affliction [suffered in this life on earth], which is but for a moment, worketh for us a far more exceeding and eternal weight of glory; while we look not at the things which are seen, but at the things which are not seen: for the things which are seen are temporal; but the things which are not seen are eternal.” (2 Corinthians:4:17-19)
No one can either refute or improve upon that statement. Everything we can now see is temporary. None of it has existed without beginning, nor will it last forever—but our invisible souls and spirits will. We must therefore make our choices and plans in this life in preparation for the next, recognizing that our visible bodies are only temporary housing for our invisible souls and spirits. These latter are indestructible and will continue to exist eternally after the body dies and is finally dissolved in the grave. Thus, the most solemn question anyone can face is this: “Where will I spend eternity?”