Can someone who has never been a vegetarian become one mentally and spiritually?

Yes, one certainly can!
The spiritual science of Akram Vignan specifies that although physically, one may be eating non-vegetarian food, they can surely become vegetarian mentally and spiritually. How??? Let’s understand that from the Enlightened One…
The Enlightened One has provided a 4-step solution, which when applied, we can get the desired result out of it.

A Firm determination to become a vegetaria

To be aware of our mistake, in itself, is a very big thing. It means you realize and agree to the fact that eating non-vegetarian food is not right. Now, make a firm determination that you want to become a vegetarian for sure and intend in your mind that, “I want to quit eating non-vegetarian food!” This is the first stepping stone to become a vegetarian mentally.

Analyze how eating non-vegetarian food is wrong and why becoming vegetarian is goodr
Gather information on how eating non-vegetarian food is wrong or bad for you. This shall develop a deeper level of understanding within you, which will help you get rid of this bad habit soon. Inspite of the understanding, for some time your mind may still be driven towards eating non-vegetarian food. However, you continue to boost your right understanding.

Whenever the mind tries to convince us that being a non-vegetarian is fine, this right understanding will work against the craving of our mind, and gradually, the latter will diminish.

Seek Forgivenessr
In the past life, knowingly or unknowingly, one has formed strong opinions in favour of how eating non-vegetarian is good, which is why one is driven towards it today. Therefore now, every time a thought of eating non-vegetarian food arises in our mind, we should immediately ask for forgiveness from the God we have faith in; and also ask him for strength to now become a vegetarian. Each time one does so, one layer of the wrong opinion gets removed and we get closer to our aim of becoming a vegetarian.

Not protecting our flawr
Most of the times, we do not like when people target on our weaknesses. However, accepting our flaw when highlighted, is one of the surest ways to get rid of it. For example, you have already decided that you want to become a vegetarian. Yet, when your friend or anybody else points out that, ‘Hey, you eat non-vegetarian. How cruel?!’, at that time, it is very crucial that we do not protect this wrong habit of ours. If you accept that, “Yes, it is definitely wrong on my part, and I’m really trying hard to get rid of this habit”, then you are said to be on your path of becoming a vegetarian soon.

This acceptance of our mistake and registering our opposition towards it, suggests that now, we are mentally and spiritually ruling over our weakness, and it’s not the other way round any more. Or else, we are internally protecting and supporting our weakness, which actually extends the habit that we are wanting to get rid of, for long.

The whole process of the above four steps is very scientific. So keep on with it as it is working beautifully inside, and shall soon create wonders in your life!
Also spiritually, becoming a vegetarian is very important!
The Enlightened One explains:

God resides within every living being. Thus, if you kill any living being, then it will be hurt immensely, and you will incur a liability for that. And this will in turn give rise to an avaran (veil of ignorance over the Knowledge of the Soul, the real Self) for you, and you will have to regress terribly towards a lower life-form. By killing other living beings, even the intellect deteriorates.

Non-vegetarian food is heavy and it does not allow the development of one’s spiritual intellect. If you want to progress spiritually, you must eat vegetarian food, which is light and does not create intoxication. It also helps increase awareness.

Spiritually, (being non-vegetarian) layers of veils get built over the Soul. Just like if we apply tar over our spectacles, do we enjoy taking a walk with those glasses. It confuses us as it obscures our vision. Thus, it is detrimental. In spiritual matters, it is obscuring the vision of the Soul. Therefore, it is best that we leave this habit. There are so many things available for eating, with the help of which we can live well. We do not want to live our life by harming other living beings.

And in today’s world, people have gone even beyond vegetarian, as they have started being vegan. These days, the vegetarian societies and vegan groups, etc. are on the increase because people have understood that these things are beneficial; non-vegetarian food is not benefitting. They are looking at it from a worldly viewpoint, and we are seeing it from a spiritual standpoint.

How is this like? Do we ever try cutting our finger purposely with a knife? No! How miserable do we feel? So when that being’s head is cut, how miserable he must be feeling? It undergoes so much of suffering, it feels torturous, it becomes so fearful, it is in lot of pain, it has lot of suffering while dying. And later, as we consume those atoms full of suffering, then fear, pain and suffering arises in us too. No one can ever become happy by giving pain to others. Therefore, do not cause pain to others. Eat vegetarian dishes. There is less risk in it. In that, it is violence of a five-sensed living being. Therefore, it is full of loss; it makes us suffer.

Hence, it is best if you do not eat it. But if you cannot stop eating it, then you should maintain the intention, ‘This is wrong, it is best to stop eating this.’ And daily ten minutes, ask for strength to that effect, “Oh Dada Bhagwan, please bless me so that I can get rid of these bad habit.”

Every morning, pray to whichever God you believe in, by saying, “May no living being be caused slightest hurt through my thoughts, words or actions. Oh Lord, make everyone free of himsa (violence).” Nurture this intention.

To know more: https://www.dadabhagwan.org/path-to-happiness/spiritual-science/non-violence-and-spiritual-awareness/food-and-spiritual-awareness/