What is the ruling on calling to “the unity of religions”?

Two things get confused here and need separating.

The first: the claim that all religions are the same — that Islam, Judaism and Christianity are equally valid paths and that the difference between them means nothing. This is rejected, because it undoes an explicit text:

“And whoever desires other than Islām as religion - never will it be accepted from him, and he, in the Hereafter, will be among the losers.” (Qur’an 3:85)

This is not a novel stance or a matter of opinion; it is what the testimony itself entails. Whoever says all paths are accepted has undone the meaning of “there is no god but Allah, Muhammad is the Messenger of Allah.”

The second: coexistence, kindness and justice with the people of other religions, and this is permitted by text:

“Allah does not forbid you from those who do not fight you because of religion and do not expel you from your homes - from being righteous toward them and acting justly toward them. Indeed, Allah loves those who act justly.” (Qur’an 60:8)

The verse does not merely permit refraining from harm; it names righteousness and equity, which are above that.

The upshot of the distinction:

• To be a neighbour to a non-Muslim, treat him well, keep him safe, deal justly with him and maintain ties if he is a relative: commanded.
• To attend with him, eat of his permitted food and exchange gifts: allowed.
• To affirm that his religion is a path accepted by Allah as Islam is: this is what is rejected.

One thing to be alert to: this principle must not become a pretext for harshness. Many who have recently accepted Islam suppose that holding Islam to be the truth obliges them to be rough with their families and neighbours. It does not, and the two verses above are read together rather than apart.