Question
Is it true that one can not convert to Hinduism from any other religion or vice versa?
Answer
No, there is no bar in converting from any religion to Hinduism or vice-versa.
Vedas speak about Mlecchas as someone who is not Vedic and not Indian. Basically, mlecchas are people who speak an illegible language and not Sanskrit. The concept of geography, religion and language was all mixed with each other to be indistinguishable.
Hinduism doesn't have a conversion ritual unlike similar ceremony in Christianity or Islam which causes the inadvertent feeling of disbelief in the believer about his conversion.
Conversion only requires that person to accept the Hindu way of life and leading a Dharmic life.
Ref 1. Mlecchas in Early India
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