I was always under the impression it was based on the entire Bible, but I seem to meet a lot of Christians who feel that the OT doesn't apply to them, only the Jews. What say ye?
What an interesting question. From the beginning, God spoke of the enemy, satan's head being crushed by Eve's (and Adam's) lineage. The entire Bible is a tapestry of God's redemption and how that comes to pass. It comes full circle in Jesus the Christ's death, resurrection, and sitting at the right hand of the Father, and His second coming, and all else that is foretold in Revelation. One part of the entire Bible cannot be omitted, for it is a circle of completion. Regardless of what one might hear, reading and studying the entire Bible on one's own is a blessing of an experience, and one misses out on verses such as these:
"The LORD thy God in the midst of thee is mighty; he will save, he will rejoice over thee with joy; he will rest in his love, he will joy over thee with singing."
~Zephaniah 3:17
Who would'a thought that this Alpha and Omega, Creator, Almighty God, the One Most High, would "rejoice" over is with "singing"? Yet there it is right there. Or who would'a thought that He comes down on those who "fake" it and bring Him offerings yet they are full of it and He knows it? Like in this verse:
"o what purpose is the multitude of your sacrifices unto me? saith the LORD: I am full of the burnt offerings of rams, and the fat of fed beasts; and I delight not in the blood of bullocks, or of lambs, or of he goats. When ye come to appear before me, who hath required this at your hand, to tread my courts? Bring no more vain oblations; incense is an abomination unto me; the new moons and sabbaths, the calling of assemblies, I cannot away with; it is iniquity, even the solemn meeting. Your new moons and your appointed feasts my soul hateth: they are a trouble unto me; I am weary to bear them. And when ye spread forth your hands, I will hide mine eyes from you: yea, when ye make many prayers, I will not hear: your hands are full of blood. Wash you, make you clean; put away the evil of your doings from before mine eyes; cease to do evil; Learn to do well; seek judgment, relieve the oppressed, judge the fatherless, plead for the widow." ~Isaiah I:11-17
Ahem, this applies to our modern day churches (us). God is looking on the heart, and He finds it insulting, and "sickening" even with vain words that are said, the prayers that are said but not meant and not even "for" or "to" Him, and yes, the "singing" that don't mean a hill o' beans cuz it's not from a heart that loves or glorifies Him.
Or who would'a thought that God says this about "fasting":
"Is not this the fast that I have chosen? to loose the bands of wickedness, to undo the heavy burdens, and to let the oppressed go free, and that ye break every yoke? Is it not to deal thy bread to the hungry, and that thou bring the poor that are cast out to thy house? when thou seest the naked, that thou cover him; and that thou hide not thyself from thine own flesh? Then shall thy light break forth as the morning, and thine health shall spring forth speedily: and thy righteousness shall go before thee; the glory of the LORD shall be thy rereward. Then shalt thou call, and the LORD shall answer; thou shalt cry, and he shall say, Here I am. If thou take away from the midst of thee the yoke, the putting forth of the finger, and speaking vanity; And if thou draw out thy soul to the hungry, and satisfy the afflicted soul; then shall thy light rise in obscurity, and thy darkness be as the noonday: And the LORD shall guide thee continually, and satisfy thy soul in drought, and make fat thy bones: and thou shalt be like a watered garden, and like a spring of water, whose waters fail not." ~Isaiah 58:6-11
Or this phenomenal verse where God spells out in His inexplicable simplicity what is "required":
"He hath shewed thee, O man, what is good; and what doth the LORD require of thee, but to do justly, and to love mercy, and to walk humbly with thy God?" ~Micah 6:8
In Proverbs (God's book of wisdom) it speaks of seeking wisdom and understanding and the blessing of finding it (this is God "blessing" us with what we seek, just as He did with Solomon, and Jabez)
"My son, if thou wilt receive my words, and hide my commandments with thee; So that thou incline thine ear unto wisdom, and apply thine heart to understanding; Yea, if thou criest after knowledge, and liftest up thy voice for understanding; If thou seekest her as silver, and searchest for her as for hid treasures; Then shalt thou understand the fear of the LORD, and find the knowledge of God. For the LORD giveth wisdom: out of his mouth cometh knowledge and understanding. He layeth up sound wisdom for the righteous: he is a buckler to them that walk uprightly. He keepeth the paths of judgment, and preserveth the way of his saints. Then shalt thou understand righteousness, and judgment, and equity; yea, every good path. When wisdom entereth into thine heart, and knowledge is pleasant unto thy soul; Discretion shall preserve thee, understanding shall keep thee:" ~Proverbs 2:1-11
And then there's Psalms, with David as an example of God's grace, forgiveness, restoration, and even God calling David the "apple of His eye". Why did He call David that? It's because of this: David loved God with all his heart, mind, soul, and strength, and even danced before Him with all his might in the sight of everyone. I suspect God, right then, was rejoicing over David with singing.
The Old Testament is a blessing of a study. In it are descriptions of God's heart, and how He carries out His promise to bring forth HIs only begotten Son, for the purposes of reuniting us with Himself, through the sacrifice of His Son, so that once more, we can walk in the cool of the day, with Him in a garden, just as it once was, before the fall.
The entire Bible is a beautifully exquisite tapestry of epic proportions that comes full circle in Revelation.