When was before the foundation of the world?

Ephesians 1:4 and 1 Peter 1:20 and numerous other passages in the Bible (Matt. 13:35; 25:34; Luke 11:50; John 17:24; Heb. 4:3; 9:26; Rev. 13:8; 17:8; cf. Rom. 8:29; 9:23; Eph. 1:11; 2 Tim. 1:9, etc.) use the phrase, "before the foundation of the world," or similar wording. When in time was this?

The word "foundation" (Greek, katabole) refers to a laying down or establishing something; depositing, sowing, deposit, technically used of the act of conception. In Ephesians 1:4 and 1 Peter 1:20, this refers to creation itself (Gen. 1:1-2:25; Job 12:7-10; Psa. 19:1; 95:4-5; John 1:3; Rom. 1:20; Heb. 11:3, etc.). The word "before" (Greek, pro) means in front of or earlier than. Though there was no literal time before creation, the term "before" sets the period, that is before establishing or creating the universe; in other words, in eternity past (cf. "I AM WHO I AM," Exod. 3:14; Deut. 33:27; Job 36:26; Psa. 90:2; 93:2). So, while we don't know the exact period, we do know that it was before the first day of creation, the creation of mankind, and the creation of baby Jesus; that is when God the Son took to himself a human body of flesh (Luke 2:7; John 1:14; cf. 1 John 4:2; 2 John 1:7).

Paul wrote in Ephesians 1:4, "even as he chose us in him before the foundation of the world, that we should be holy and blameless before him. In love" ... (cf. 2 Tim. 1:9). Paul states that even before establishing the world itself, which includes mankind on Day 6, that God in love choose all those that would ultimately believe in His only begotten Son Jesus Christ. Note that the word "chose" doesn't mean to give a mere opportunity for something, rather it means an actual choice of a certain thing; He chose the wheat, and not the tares (Matt. 13:24-30), the sheep, and not the goats (Matt. 25:31-46), and those whom by his grace alone would eventually be saved, and not remain lost (Luke 15:1-31). In other words, "all who were appointed for eternal life believed" (Acts 13:48). Even His people's good works were prepared in advance (Eph. 2:10). The is nomally called the doctrine of "election" and / or "predestination." Please see, "Calvinism and Matthew 13?" below.

So, the Christian's salvation does not begin with their choice to believe in Christ (John 15:16, 19), but in eternity past. Our salvation began before the creation of the universe (Eph. 1:4), when God planned the history of redemption - including the birth, life, death, resurrection, and ascension of his only begotten Son - and chose His people to be His own through Christ alone. What great assurance this should give to the believer; all those predestinated will definitely be glorified (Rom. 8:29-30).

So, before the omniscient (all-knowing), omnipotent (all-powerful), omnipresent (being everywhere) God created anything at all, numerous other things had already been decided by God: (1) humanity would be created "very good" (Gen. 1:31); (2) humanity would sin and be lost (Gen. 3); (3) the plan of redemption and the Redeemer were ordained/chosen (Rom. 5:12-19; 1 Pet. 1:20); and (4) Jesus, the second and last man Adam (1 Cor. 15:45, 47), would only die for and save those whom God choose before the foundation of the world (Eph. 1:4; 2 Tim. 1:9; 2:10). Actually, many other things were before the foundation as well, such as;

(1) God existed; He is eternal (Gen. 1:1; Psa.90:2; 1 Tim. 1:17).

(2) The Son of God and the Spirit existed (Isa. 48:16; Mic. 5:2; John 1:1; 17:5, 24).

(3) The Trinity existed ("us" and "our" in Gen. 1:26).

(4) The Word existed/is eternal (John 1:1).

(5) Wisdom was established (Prov. 8:23).

(6) God promised eternal life (Tit. 1:2).

(7) Christ was foreordained to die and rise again (1 Pet. 1:20-21).

(8) Hidden secrets of God existed (Mat. 13:35).

(9) The kingdom was prepared for God's people (Mat. 25:34).

(10) The blood of all the prophets was shed (Luke 11:50).

(11) Certain people's names weren't written in the Book of Life (Rev. 13:8; 17:8).

And while I realize I left out a lot (time nor our limited knowledge doesn't permit us to list everything that was done before the foundation of the world), we can biblically say a lot happened before time even existed!

The other part of your question mentioned 1 Peter 1:20. Peter wrote, "He [Jesus] was foreknown before the foundation of the world but was made manifest in the last times for the sake of you" (1 Pet. 1:20; 1 Cor. 2:7). Jesus was born to die. Please see, "Was Jesus born only to die?" below. As Jesus stated in his High Priestly Prayer, "Father, I desire that they also, whom you have given me, may be with me where I am, to see my glory that you have given me because you loved me before the foundation of the world" (John 17:24). Note how Jesus links those chosen before the foundation of the world (Eph. 1:4) with God's love and Himself before the foundation of the world (1 Pet. 1:20).

The Second Person of the Trinity became a man; God the Son became incarnate becoming the God-Man (John 1:14; Phil. 2:6-8; Heb. 2:14): (1) Jesus has two natures; He is God and man; (2) each nature is full and complete; He is fully God and fully man; (3) each nature remains distinct; (4) Christ is only one Person; and (5) things that are true of only one nature are nonetheless true of the whole Person of Christ. So, Jesus "has" (present tense) a genuine human body, a will, a mind, a heart, real hands, feet, eyes, ears, and a mouth, etc. (John 1:14). See "The Virgin Birth: Jesus' Chromosomes and DNA?" below. He literally lived, died, and resurrected from the dead (Acts 2:23-27; 4:27-28; 1 Cor. 15:1-4; 1 Tim. 3:16). He ascended to the right hand of God (Mark 16:19), and lives forevermore (Heb. 7:24).

Therefore, it necessarily follows that God must have ordained absolutely everything. Why? Because everything created is interrelated to everything else created (a web of multiple reciprocities), so without the divine appointing of all things, the plan of the Redeemer could have been interrupted, even stopped; many attempts were made (Matt. 2:13; Luke 4:1-13, etc.)! However, He came in the fullness of time (Gal. 4:4; cf. Mark 1:15; Rom. 5:6; 1 Tim. 2:6) and finished what He was sent to accomplish (John 4:34; 17:4; 19:30). Please see, "Is God sovereign over every second of time" below.

Related Links:

Calvinism and Matthew 13?
Was Jesus born only to die?
The Virgin Birth: Jesus' Chromosomes and DNA?
Is God sovereign over every second of time?