Other religions may practice a baptism of sorts, but it is not Christian (i.e., Catholic) baptism unless it follows the correct form (invoking Father, Son, and Holy Spirit) and with the intent to do what the Church intends.
A baptized Christian who becomes Hindu is technically an apostate, but retains the indelible mark of his Christian baptism. So he is still a Christian in a sense, but a “lost sheep” for whom the Father patiently waits…
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