GENESIS 1:7

Thus God made the firmament, and divided the waters which were under the firmament from the waters which were above the firmament; and it was so. [Genesis 1:7 NKJV]
 

And God made the expanse [of sky] and separated the waters which were under the expanse from the waters which were above the expanse; and it was so [just as He commanded]. [Genesis 1:7 AMP]

Bible Teacher:

God did not just make the Firmament in the midst of the waters, but He also divided the waters under the Firmament from the waters above the Firmament. He made the Firmament to be inside the waters, but made the waters inside the Firmament to come under the Firmament. The Firmament ruled the waters under it.
 

Are you like the Firmament in the hand of God? If you are, you may not have been called to rule all the existing waters or to rule from the top; but God called you and empowers you to rule from midst the waters and to rule the waters within you by bringing them under your subjection and control. He who rules his own spirit and brings his body under subjection is better than he who captures a city [Proverbs 16:32][1 Corinthians 9:27].

Commentaries:

The firmament of separation began the division or splitting of the worlds or realms. God was so determined and willing to divide the waters from the waters that He both spoke His will and made the firmament to practically fulfill that purpose: and it became a lasting ordinance. So much was invested by God into separating the waters below from their fellow waters above the Firmament. However, much more have been invested by God into separating His People above from their sin and death below through the offering of God's Son.

Bible Teacher:

Note: the Bible verse above did not say God divided/separated the waters above from the waters below the Firmament; rather it said God divided/separated the waters below from the waters above the Firmament. This meant the separation was not by exalting the waters above further, but by demoting or pushing down the waters below further because the Firmament expanded from top to bottom, not from bottom to top, creating more depths rather than creating more heights for the separated waters. Hence, angle of depression applied instead of angle of elevation.

Commentaries:

God made the Firmament, and what the Firmament did was what God did through it: and that remained so until God would change that. 
 

• When God made the Firmament He planted or made a heaven. [Genesis 1:8]

• When He separated/divided the waters, He stretched the heaven like a curtain. [Isaiah 40:22]
   
• He used the waters under the Firmament, called seas, as the Foundation of the Earth. [Psalms 24:1-4, Genesis 1:9-10]
   
• He used the waters above the Firmament as Base for the Beams of His Chambers. [Psalms 104:2-3]
   
• And it was so, and stood so in lasting ordinance, till when God would change that as vesture. [Hebrews 1:10-12]
   

[Zechariah 12:1 NKJV] — Thus says the Lord, who stretches out the heavens, lays the foundation of the earth, and forms the spirit of man within him:

[Isaiah 42:5 NKJV] — Thus says God the Lord, Who created the heavens and stretched them out, Who spread forth the earth and that which comes from it, Who gives breath to the people on it, And spirit to those who walk on it:

Bible Teacher:

But consider the separation of the waters below from the waters above the Firmament: the molecules of waters below the Firmament (Heaven) were separated from the molecules of waters above the Firmament, by God using the Firmament. The same waters, but those molecules that be below the midst or firmament were further abased and separated from those molecules that were above the Firmament. People are figuratively like waters: Some people have their minds on the things above in heavens where Christ is sitted with God, but other people have their minds on the things below on this earthly realms. Behold, there shall be separation between the people who's affection is above from the people who's affection is below when Christ shall appear, as there was separation between the waters who's molecules were above from waters who's molecules were below when the Firmament appeared. As it is written:

[Colossians 3:1-5 BSB'19] — "Therefore, since you have been raised with Christ, strive for the things above, where Christ is seated at the right hand of God. Set your minds on things above, not on earthly things. For you died, and your life is now hidden with Christ in God. When Christ, who is your life, appears, then you also will appear with Him in glory. Put to death, therefore, the components of your earthly nature: sexual immorality, impurity, lust, evil desires, and greed, which is idolatry."