He was not the first to say so - starting from 1000 CE till today 2018 CE, there have been numerous Gurus who have been saying the same thing in different ways over and over again and ignoring the predatory nature of these other faiths and the empirical reality of a shrinking dharmic population.
Every guru thinks that he will be the one to "solve" the dharma-religion conundrum. Not only does it not happen - what happens is that we become weaker and weaker as we compromise our dharmic tenets and try to "accommodate" them in the name of "harmony" and "syncretism".
But its a one way traffic.
Ramakrishna Mission worships Christ and celebrates Christmas, but Vatican or Protestant churches will NEVER even entertain the idea of worshiping Krishna and celebrating Diwali.
Many Gurus say absurd things like AUM is same as Ameen and Amen. They say that Christians/ Muslims doing Yoga can replace AUM with Amen/ Ameen. It is ridiculous. No Christian or Muslim will every say that Amen is AUM or Ameen is AUM. And they don't even mean the same thing - AUM is an auspicious sound and Amen/ Ameen means "so be it" or tathastu. It is insulting to them also when we try to interpret their religion for them, as Rajiv Malhotra has said many times.
Sri Sri Ramakrishna is said to have merged with Jesus and Allah. I have studied Christianity and Islam in depth and found it very very hard to digest the way it has been narrated. In fact there are questions if there was ever a historical Christ. People will say that our mental plane and Sri Ramakrishna's mental plane is not the same. He spoke from the depth of Divinity whereas we speak from a materialistic, statistical and political viewpoint. That Ramakrishna went beyond religion. He went to a point where books, dogmas, churches or temples are unnecessary. So it is difficult for us to understand him.
Such answers will not do. We need logic and rigor. Suppose it is proved 100 years from now that there indeed was no Jesus Christ, then how will the narration be reconciled? Blind supporters of Ramakrishna Mission say that to question Sri Sri Ramakrishna Paramhamsa is blasphemy. That the existence of Christ is not an issue here. That it is the foolishness of some Christians who take everything in the Bible quite literally. That Jesus is an idea - of purity and divinity that shall help mankind to cross the ocean of causation and misery. The mature reader will not go in for archeological evidence and such stuff because spirituality is for the spirit.
Blasphemy? Really? First of all, Kathamrita is a hagiography - what devotees wrote. And devotees always tend to exaggerate and talk about miracles. Secondly, there is no concept of "spiritual Jesus" in Christianity proper/ Church. It is an idea developed by Hindus to accommodate Christianity - which is my point that our Gurus Hinduize everything. Thirdly, Pope and various Protestant churches and seminaries are the authority on Jesus - for them Jesus was a real (although not yet proved) person. They take everything literally - there are Biblical- Jesus tours. Who are we to question their tradition and say Jesus is an idea?
I am not denying his merging with Jesus - except that the Jesus he merged with was his own interpretation/ imagination of Jesus from a sanatan lens - not the Jesus of Christianity as practiced by 220 crores Christians worldwide. Because in Christianity one cannot merge with Jesus. That is the real blasphemy. For them creator is separate from creation - not all-pervading.
Pramhansa was at Paramarthik level where no duality exists. All of us are at Vyavharik level where we are facing issues like conversion and attacks. In this realm, one cannot apply ideas of a Paramhans. We have to play our role, oppose Adharma. We cannot runaway from duty citing examples of Ramakrishna simply because we have not yet reached his level.
We have clearly forgotten the key message of both the Gita and Adi Shankara's life. Gita is very clear about what to do with adharma. Krishna never says that you kill Kauravas selectively - his advice was categorical - we have tried all methods, the only option now is to finish off the adharmins in totality. Adi Shankara did that intellectually - he decimated his opponents and converted them. He established dasanami order to protect dhrama. He never said (at least I have not read anywhere) - let us do Buddha Puja. And because of his clear framework Sanatan dhrama survived till we started compromising.
Then why do we do Jesus Puja and all that Ishvar-Allah nonsense? Is it ever reciprocated?
Why do we need to at all be western compliant? Why at all bring Jesus or Allah into the picture? Why don't we stick to our tradition? Why don't we have the guts to say - "this our dharma, take or leave it. I have not come here to make it science compliant or Christ compliant or secular compliant"? Why are we so defensive about our dharma?
The empirical data is clear. Hindus have been steadily declining as a percentage of total South Asian population [100% to 65%] - for the past 1000's years. This is despite numerous attempts at "syncretism", universal religion, sarva dharma sambhava, "jata mat tata path" and what not. It is going to go down further. Clearly one way universalism does not work - Hindu gets compromised while the Muslim/ Christian not only do they not Hinduize they become more "orthodox". Now people will give different reasons for the decline - but the key point is, the numbers are real and efforts at "harmonizing" have failed miserably.
It is foolish to say, I accept Jesus but not the Church or that I accept Allah and not Mohammed. They are package deals - Bible, Jesus, Church and Koran, Allah, Mohammed - it is our foolishness to think that we can selectively pick and choose from a stronger enterprise. The reverse is not true as we are weak and they continue to dismantle our dharma piece by piece - yoga, ayurveda, tantra, sadhana and so on.
Clearly we are the losers and we will continue to lose as long as we continue to compromise.