Are there any Christian churches or groups which teach that all religions are inherently good and that one need not convert to Christianity and that it is enough for an adherent to remain in their non-Christian religion provided that they live faithfully by it?
All Christian churches and groups that base their beliefs on the teachings of Emanuel Swedenborg (1688-1772) believe that people of all religions can be saved if they believe in God and live a good life according to the teachings of their own religion. This belief is not a recent development, but has been part of New Church (Swedenborgian) doctrine since the church was first organized as a distinct sect in the late 18th century, a decade and a half after Swedenborg's death.
Here is an example of Swedenborg's teaching on this subject:
It is an insane heresy to believe that only those born in the church are saved. People born outside the church are just as human as people born within it. They come from the same heavenly source. They are equally living and immortal souls. They have religions as well, religions that enable them to believe that God exists and that they should lead good lives; and all of them who do believe in God and lead good lives become spiritual on their own level and are saved. (Divine Providence #330.5)
By "the church" in this context Swedenborg means the Christian church as a whole, including all denominations and branches.
Swedenborg taught that all religions are provided by God, and that there are different religions because religion must be adapted to the culture and character of the different nations and peoples of the world. For more on this subject from a Swedenborgian perspective, see: If there’s One God, Why All the Different Religions?
Swedenborgians hold to this belief while themselves believing in a strongly Christian theology. Swedenborg taught that the salvation accomplished by Jesus Christ through his life, death, resurrection, and ascension made salvation possible for all people, not only Christians, and that those who believe in God are in fact believing in Jesus Christ because Jesus Christ is God, and there is no other God to believe in. For more on this, see Who is God? Who is Jesus Christ? What about that Holy Spirit? and Is Jesus Christ the Only Way to Heaven?