Why does the Orthodox Church Chant the Psalms?

Why are all her hymns, songs, prayers and readings either Holy Scripture, or prose and poetry completely reflected in Holy Scripture?   

Because David and Moses spoke divine worship into creation via the power of the Holy Spirit, giving us God's pattern of worship. The "place has change" from the tabernacle to the temple, from the temple to the synagogue, from the synagogue to the hearts of those who believe, those who now gather holding the Cross of Christ ever before them.   John 4:23-24 "But the hour cometh, and now is, when the true worshippers shall worship the Father in spirit and in truth: for the Father seeketh such to worship him. 
God is a Spirit: and they that worship him must worship him in spirit and in truth." 

When they gather they do not ape the patterns of the culture of any age, which would offer to God what he has not ordained HOLY and Worthy as Divine Worship. You will hear non-orthodox comparing their worship to that "enthusiasm" given rock-stars or sports events. Sometimes their worship actually mimics these events. This is NOT the Divine Worship of the Christ's Church.  
The Church gathers WITH the Patriarchs, Prophets, and Saints of all ages who with the Holy Angels surround the Throne of God, and sings with them, "The heavenly choir sings Your praises, saying: 
"Holy, Holy, Holy, Lord of Sabaoth, Heaven and earth are full of Your glory. 
"Come to Him and be enlightened and your faces shall not be ashamed.
"The heavenly choir sings Your praises, saying, "Holy, Holy, Holy, Lord of Sabaoth, Heaven and earth are full of Your glory." 
"+Glory to the Father and to the Son and to the Holy Spirit.
"The choir of holy angels and archangels, with all the powers of Heaven, sings Your praises, saying "Holy, Holy, Holy, Lord of Sabaoth, Heaven and earth are full of Your glory." 
"Now and ever and unto ages of ages. Amen.

Rather they gather by the Power of the Holy Spirit Who enlightens their "NOUS" (Greek = their spiritual capacity, sometimes translated "heart," sometimes translated "mind") drawing them to this "service" (Literal translation Liturgy) so that Christ the Head of the Body, may share with them his Sacrifice, His Body and His Blood and they BECOME in that Worship the "living sacrifice" joining with the sacrifice of Jesus, Himself, that sacrifice Saint Paul described in Romans the 12th Chapter, they present themselves, prepared for the Liturgy, to be that living sacrifice, holy and acceptable which is their "reasonable service" (Literal Translation which is your "Rational Liturgizing") 2  And be not conformed to this world: but be ye transformed by the renewing of your mind, that ye may prove what is that good, and acceptable, and perfect, will of God.  

John 6:53 (KJV) Then Jesus said unto them, Verily, verily, I say unto you, Except ye eat the flesh of the Son of man, and drink his blood, ye have no life in you." 



 "1 Corinthians 10:16 (KJV) This cup of blessing which we bless, is it not the communion of the blood of Christ? The bread which we break, is it not the communion of the body of Christ?" 

The Divine Liturgy

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