Question: "How interested are Christians supposed to be in the spirit world?"
Answer: The simple answer to this question is “very interested.” A human being is comprised of body, soul, and spirit (1 Thessalonians 5:23; 3 John 1:2; Psalm 16:9). However, human beings tend to rely on the body for input and the soul for decisions, while ignoring the spirit. This is unfortunate. The human spirit without God is like a deflated balloon. When a sinner repents and turns to Jesus for salvation, God sends His Holy Spirit to dwell within the spirit of that believer (Luke 24:49; John 14:26; 1 Corinthians 6:19). The Holy Spirit breathes life into that deflated human spirit, and a new creature is born (2 Corinthians 5:17). The more room a person gives to the Holy Spirit, the more power he or she experiences in living for God.
God is spirit (John 4:24). If we want to know God, we must experience Him spiritually. Although God works in tangible, physical ways through His creation (Psalm 8:3; 107:24), we come to know Him personally through the union of our spirits with His (Romans 8:16). As we allow the Holy Spirit free rein in our lives, we learn to live by the Spirit, rather than by emotion, impulse, or fleshly indulgence (Galatians 5:16, 25; Romans 8:14). We learn to discern the voice of God as distinct from our own thoughts (John 10:27). All of this takes place within the spirit, invisible to the other senses, but is as real as touch, taste, or smell.
However, the term spiritual does not necessarily mean “godly.” Satan is also a spirit and does his evil work by attacking our minds (James 3:14-15), our bodies (Luke 9:42; 2 Corinthians 12:7), and our spirits (Matthew 16:23; 2 Corinthians 10:3-5). Some have delved into an exploration of the spirit world to their own destruction. The seven sons of Sceva are a case in point. They were assuming a knowledge of the spiritual realm and an authority they did not possess. They learned the hard way that spiritual warfare is not to be taken lightly; it can only be fought successfully by those who are in Christ and equipped for battle (Acts 19:13-16). Also, many people consider themselves "spiritual" while completely bypassing the true God who is the King of the spirit world (Mark 3:11). Such people are deceived by the “god of this age [who] has blinded the minds of unbelievers” (2 Corinthians 4:4).
The Bible is clear that the spirit world is every bit as real as the physical universe (Ephesians 6:12). There is an unseen battle taking place around us every moment between God's holy angels and the forces of darkness (Daniel 10:12-14; Ephesians 6:10-17; Jude 1:9). If we are vigilant as the Lord commands, we will not be caught unprepared by Satan’s attacks (1 Peter 5:8; 2 Corinthians 2:11). And we have the promise of God that His Holy Spirit is stronger than any of Satan's schemes (1 John 4:4). God has given His children everything we need to stand firm against any spiritual attack of our enemy. The apostle Paul calls this the "armor of God" (Ephesians 6:11).
The spirit world is very real, but an unbalanced focus on demonic powers is not healthy and does not glorify God. The Holy Spirit is the only Spirit we should ever invite into our lives, and He has all the power we need to overcome anything Satan may use to try to defeat us (Isaiah 54:17).