(Is not He Who has made the earth as a fixed abode,) meaning, stable and stationary, so that it does not move or convulse, because if it were to do so, it would not be a good place for people to live on. But by His grace and mercy, He has made it smooth and calm, and it is not shaken or moved. This is like the Ayah,

(Allah, Who has made for you the earth as a dwelling place and the sky as a canopy) (40:64).

(and has placed rivers in its midst,) means, He has placed rivers which are fresh and sweet, cutting through the earth, and He has made them of different types, large rivers, small rivers and some in between. He has caused them to flow in all directions, east, west, south, north, according to the needs of mankind in different areas and regions, as He has created them throughout the world and sends them their provision according to their needs.

(and has placed firm mountains therein, ) means, high mountains which stabilize the earth and make it steadfast, so that it does not shake.

(and has set a barrier between the two seas) means, He has placed a barrier between the fresh water and the salt water, to prevent them from mixing lest they corrupt one another. Divine wisdom dictates that each of them should stay as it is meant to be. The sweet water is that which flows in rivers among mankind, and it is meant to be fresh and palatable so that it may be used to water animals and plants and fruits. The salt water is that which surrounds the continents on all sides, and its water is meant to be salty and undrinkable lest the air be corrupted by its smell, as Allah says:

(And it is He Who has let free the two seas, this is palatable and sweet, and that is salty and bitter; and He has set a barrier and a complete partition between them.) (25:53) Allah says:

(Is there any god with Allah) meaning, any god who could do this, or who deserves to be worshipped Both meanings are indicated by the context.

(Nay, but most of them know not!) means, in that they worship others than Allah.