The Garden of Eden, God's perfect world, was vegetarian (Gen. 1:29-30). Immediately, God calls this ideal and non-exploitative relationship "good" (Gen. 1:31). This is the one time when God makes such a statement. There follow many years of fallen humanity, when people held slaves, waged war, ate animals, and committed various other violent acts. Although there are passages in the Scriptures that endorse eating animals, war, slavery, polygamy, animal sacrifice, and other practices that most people find immoral, these passages are a representation of what existed as a part of fallen humanity, not of God's ideal plan or vision.
Complementing the new focus on baptism were opposition to the Temple, where the animals were sacrificed, and celebration of Passover without the lamb (slaughtered sacrificially in the Temple). In fact, Jesus rails against the Temple, and his single act of direct confrontation with the authorities is in the Temple, the slaughterhouse of first century Palestine, when he engages in direct action by casting out all those selling animals for sacrifice.You can debate his reasons, but the practical effect was a Jew preventing others Jews from engaging in the Passover slaughter they felt was called for by God. Clearly, Jesus rejects this notion, claiming twice that they should all learn the meaning of Hosea when he says, speaking for God, "I desire mercy, not sacrifice."
Jesus calls multiple fishermen away from their occupation of killing animals and pleads with them to show mercy to all beings, quoting Hosea again "I desire mercy and not sacrifice." In each instance, they immediately abandon their occupation of fishing to follow Jesus (e.g., Matthew 3, Mark 1, and Luke 5). This resembles Jesus' call to tax collectors, prostitutes, and others, who are engaged in activities that are not in line with his message of mercy and compassion.
One final point to make is that many Christians of the first three centuries, including all of the "Desert Fathers," were vegetarians and that they continued, despite being Jewish, to celebrate the Passover with bread and wine, not lamb's flesh. It would be strange indeed for the first Christians to have adopted a diet not followed by Jesus himself.
"Animals are God's creatures, not human property, nor utilities, nor resources, nor commodities, but precious beings in God's sight. ... Christians whose eyes are fixed on the awfulness of crucifixion are in a special position to understand the awfulness of innocent suffering. The Cross of Christ is God's absolute identification with the weak, the powerless, and the vulnerable, but most of all with unprotected, undefended, innocent suffering."
-Rev. Andrew Linzey
"Not to hurt our humble brethren (the animals ) is our first duty to them, but to stop there is not enough. We have a higher mission - to be of service to them whenever they require it... If you have men who will exclude any of God's creatures from the shelter of compassion and pity, you will have men who will deal likewise with their fellow men."
-St Francis of Assisi
It is a heart which is burning with love for the whole of creation, for men, for the birds, for the beasts... for all creatures. He who has such a heart cannot see or call to mind a creature without his eyes being filled with tears by reason of the immense compassion which seizes his heart; a heart which is softened and can no longer bear to see or learn from others of any suffering, even the smallest pain being inflicted upon a creature. That is why such a man never ceases to pray for the animals... (He is) ... moved by the infinite pity which reigns in the hearts of those who are becoming united with God.
-St Isaac the Syrian, one of the early Church mystics, when asked the question "What is a charitable heart?":
"It may be suggested by some books that it is not a sin to kill an animal, but it is written in our own hearts - more clearly than in any book - that we should take pity on animals in the same way as we do on humans."
-Leo Tolstoy