Vegetarian diet contributes to Spiritual development while Meat consumption contributes to Spiritual degradation
There will be no real spiritual development or real devotion as long as you eat meat, enjoy the food which is prepared by harming orkilling the animals.
Here is the important question for those who love to eat meat – "Why the same Supreme God, who created both the humans and the animals (besides creating everything else in this universe), made the human body in such a way that if you scientifically evaluate the biological similarities between the man and all the types of vertebral animals, the human body is falling under the herbivorous category and not under the carnivorous category?"
Again in this context of biological similarities, we are only referring to the book "The Higher Taste" by ISKCON as it contains this information which is not only speaking about practices of Vedic culture of ancient India but also about the scientific facts established and rediscovered by the Western world itself.
Some religions say that God has created all the animals for man so that he can eat them. If it is not forbidden in their religious books, they need not exclude anything from their meal. Some religions only abstain from eating the meat of some animals like pigs or cows or both. But 'meat eating' is 'meat eating' and there is no spiritual progress if you abstain from one particular animal's meat.
Having said this, it is to be understood that nobody can be 100% non-violent. This is because eating vegetarian diet also accounts to some violence of killing the plants and harming the trees. For this, the Vedic civilization has an explanation that for the sake of existence, if you consume the vegetarian diet or meat, it can't be considered as 'Violence' because the living beings have their right to live, and to live, they must eat something.
The Vedic civilization further explains that there is a hierarchy among all the living beings and based on the hierarchy of a particular species, you accumulate sin when you kill the living beings. By comparison, adopting a vegetarian diet makes you accumulate less sin, whereas consuming meat by harming/killing animals which are of higher biological order, makes you accumulate more sin.
As a conclusion, you can offer food to the God prior to your consumption to gently appeal to the Ultimate that you are consuming the food for the sake of living. With such a noble austerity, the food offered to God becomes 'Prasada' (sacred food). No real Brahmin, who follows the Vedic culture, can offer meat to the God and eat it later as 'Prasada'.
In some countries due to the environment, it is not possible to have the vegetation suitable for consumption. In such inevitable situations, people have no other choice than to go for meat consumption.
The truth is that the Supreme God has created animals and other creatures in this world for the man to recognize the value of human life and to discover the distinction between the humans and the rest of the living creatures. So the man can start his discovery for getting answers to his questions which ultimately provides the spiritual progression and Salvation. Using animals is different from eating animals.