Is it wrong for a believing Christian to fantasize?
Let us first determine the full meaning of the word fantasy…
fan-ta-sy (fant se, -ze) n. , pl. -sies [[ME fantasie < OFr < L phantasia , idea, notion < Gr, appearance of a thing < phainein , to show, appear < IE base *bha- , to gleam, shine > OE bonian , to ornament]]
1 imagination or fancy; esp., wild, visionary fancy
2 an unnatural or bizarre mental image; illusion; phantasm
3 an odd notion; whim; caprice
4 a) a work of fiction portraying highly IMAGINATIVE (sense 3) characters or settings that have no counterparts in the real world b) such works, collectively, as a literary form; specif., those works dealing with dragons, elves, ghosts, etc.
5 Music FANTASIA (sense 1a )
6 Psychol. a) a more-or-less connected series of mental images, as in a daydream, usually involving some unfulfilled desire b) the activity of forming such images –vt. -sied, -sy-ing to form fantasies about –vi. to indulge in fantasies, as by daydreaming 
Jesus, when He gave His Sermon on the Mount Beatitude, said Matt 5:17 “Do not think that I came to destroy the Law or the Prophets. I did not come to destroy but to fulfill. Jesus reaffirms that He has not come to destroy the Law (The commandments) and what the Prophets of old had prophesied; but that He was the fulfillment of their prophecy and also the Law…Remember what Jesus told His Disciples about the New Covenant? LK 22:20 Likewise He also took the cup after supper, saying, “This cup is the new covenant in My blood, which is shed for you.
This New Covenant in Jesus’ Blood is described by the Prophet Jeremiah Jer 31:33 “But this is the covenant that I will make with the house of Israel after those days, says the LORD: I will put My law in their minds, and write it on their hearts; and I will be their God, and they shall be My people. If you are a professing Christian, you are now grafted into Israel and the Law is now in your mind and in your heart…Jesus confirms this as He teaches his Disciples on the Mount Beatitude.
Matt 5:27 “You have heard that it was said to those of old, ‘You shall not commit adultery.’
5:28 “But I say to you that whoever looks at a woman to lust for her has already committed adultery with her in his heart.
This now is fantasizing…you are not physically doing the deed, but you are putting the thoughts into sinful action in your mind through fantasy. Although Jesus speaks of the sin of adultery as an example here, it can be applied to many other aspects of our lives …like fantasizing over our neighbor’s belongings…fantasizing over Harry Potter books and the occult… the list is actually endless and usually involves some unfulfilled desire or desires. In the definitions of the word fantasy above, item 4 refers to highly imaginative characters or settings that have no counterparts in the real world …those works dealing with dragons, elves, ghosts, etc.
Don’t fantasize on things of the occult. Don’t read Harry Potter books, or associate with anything remotely occult
Don’t fantasize how you can get even with someone who has harmed you…turn the other cheek.
Don’t fantasize how great it would be to have a better spouse. Love whom you have been given.
Don’t fantasize how great things will be if you had millions in the bank. Be grateful for what you have.
Don’t fantasize by thinking anything that may be harmful, painful or derogatory to others…Keep your mind pure and uncluttered by fantasies.
Note with keen interest that Jesus is giving this lesson to His Disciples primarily, and the crowd is listening on, amazed at His teachings, Matt 5:1 And seeing the multitudes, He went up on a mountain, and when He was seated His disciples came to Him.5:2 Then He opened His mouth and taught them … So the entire teachings, or the Beatitudes, as they are commonly referred to, would appear to be directed at BELIEVERS ; I am of the persuasion that it is impossible to keep Jesus’ Beatitude teachings unless one is Born again and indwelled by His Holy Spirit …and to those who presume that they cannot lose their salvation, please read what one must do, as a believer, in order not to be cast into hell…
5:29 “If your right eye causes you to sin, pluck it out and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.
5:30 “And if your right hand causes you to sin, cut it off and cast it from you; for it is more profitable for you that one of your members perish, than for your whole body to be cast into hell.
Don’t play games with God …the consequences are diabolical. Remember that Jesus paraphrased the 10 commandments into two sentences, both starting with LOVE:-
Mark 12:30 ‘And you shall love the LORD your God with all your heart, with all your soul, with all your mind, and with all your strength.’ This is the first commandment.
12:31 “And the second, like it, is this: ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.”
These words should be engraved on our hearts and in our minds at all times…Are they in yours?
There is no better time than right now to spread the Gospel; why not do it?