51th rule of the same holy Apostles: "If any bishop, or presbyter, or deacon, or indeed any one of the sacerdotal catalogue, abstains from marriage, flesh, and wine, not for his own exercise, but because he abominates these things, forgetting that "all things were very good," Genesis 1:31 and that "God made man male and female," Genesis 1:26 and blasphemously abuses the creation, either let him reform, or let him be deprived, and be cast out of the Church; and the same for one of the laity. "
53rt rule of the Apostles: "If any bishop, or presbyter, or deacon does not on festival days partake of flesh or wine, let him be deprived, as "having a seared conscience," 1 Timothy 4:2 and becoming a cause of scandal to many."
2nd rule of the synod at Gangra (approved by the 3rd Ecumenical council): "If any one shall condemn him who eats flesh, which is without blood and has not been offered to idols nor strangled, and is faithful and devout, as though the man were without hope [of salvation] because of his eating, let him be anathema."
Apostle Paul (1 Tim 4:1-5): "Now the Spirit expressly says that in later times some will depart from the faith by devoting themselves to deceitful spirits and teachings of demons, through the insincerity of liars whose consciences are seared, who forbid marriage and require abstinence from foods that God created to be received with thanksgiving by those who believe and know the truth. For everything created by God is good, and nothing is to be rejected if it is received with thanksgiving, for it is made holy by the word of God and prayer."
14th rule of the synod at Ancyra: "As for those presbyters or deacons who are in the clergy and who abstain from meat, it has seemed right for them to touch and taste the meat and then, if they so wish, to refrain from eating it; but if they are unwilling to eat even vegetables that have been cooked with meat, and refuse to submit to the Canon, let them be dismissed from the orders."
A summary of all this: abstaining from meat as form of ascetism is good. But in order to safeguard from heresies, everyone should test his consciousnes by eating meat at least occasionally or maybe even only by eating vegetables cooked with meat.
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According to Theodoret of Cyrus this was because God knew that in the future people would worship various animals, so in this way the people would know the indecency of this.
Jerome: "But afterwards when God saw that the heart of man from his youth was set on wickedness continually, and that His Spirit could not remain in them because they were flesh, He by the deluge passed sentence on the works of the flesh, and, taking note of the extreme greediness of men, gave them liberty to eat flesh: so that while understanding that all things were lawful for them, they might not greatly desire that which was allowed, lest they should turn a commandment into a cause of transgression."