The phrase used consistently in the New Testament (every time, in fact) for the 'foundation of the world' (meaning the creation of the earth), is the 'themelios of the ktesis'. This literally means 'foundation of the creation'.
The word 'themelios' means the literal, physical foundation of a literal, physical building. The word 'ktesis' means the literal creation.
The phrase to which you have referred, translated 'foundation of the world', is the 'katabole of the kosmou'. This literally means 'the casting down of the age' - the fall, no less.
The word 'katabole' literally means 'to throw down'. It is always used in this sense. It simply does not mean 'foundation', except metaphorically - and the creation was not a metaphor. Where the foundation of the creation is spoken of, the NT invariably uses the word 'themelios', which is as literal as you could wish to get.
In just one place the NT uses the word 'katabole' in a metaphorical sense, and that is to use one of its secondary meanings, 'to sow seed'. The reference is in Hebrews, speaking of Sarah and Abraham.
The word 'kosmou', is of course the genetive singluar form of the word 'kosmos', and we know that the word 'kosmos' is not used to speak of the physical creation. Consistently, the word 'ktesis' is used when speaking of the literal, physical creation, which agrees perfectly with the word 'themelios', which also has a fundamental physical meaning.
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Matthew 13:
35That it might be fulfilled which was spoken by the prophet, saying, I will open my mouth in parables; I will utter things which have been kept secret from the foundation of the world.
Matthew 25:
34Then shall the King say unto them on his right hand, Come, ye blessed of my Father, inherit the kingdom prepared for you from the foundation of the world:
John 17:
24Father, I will that they also, whom thou hast given me, be with me where I am; that they may behold my glory, which thou hast given me: for thou lovedst me before the foundation of the world.
Ephesians 1:
4According as he hath chosen us in him before the foundation of the world, that we should be holy and without blame before him in love:
Hebrews 4:
3For we which have believed do enter into rest, as He said, As I have sworn in my wrath, if they shall enter into My rest: although [or, 'and yet']the works were finished from the foundation of the world.
Undoubtedly the work of creation was finished before the fall. However, despite this, the purpose of God (them entering into His 'rest'), could not be fulfilled subsequent to the fall, despite the work of creation itself being ready from the fall.
The contrast is between who was ready from the fall.
1) God was ready.
2) The creation (excluding Adam and Eve) was ready.
3) Adam and Eve were
not ready.