Elin,
These are the Biblical facts you have to deal with in this regard:
1) Jn 6:37-40, 10:27-29, 11:51-52; Ro 8:28-39; Eph 1:3-14; 1Pe 1:20 where
God speaks of having chosen a great number and having sent Christ to save them.
Only one text, John 6:39 is addressing the atonement. The others are addressing the purpose of the atonement.
) Jn 10:15-18, 27-29; Ro 5:8-10, 8:32; Gal 2:20, 3:13-14, 4:4-5; 1Jn 4:9-10; Rev 1:4-6, 5:9-10
where Christ is regularly said to have died for particular groups, with the clear implication that
his death has secured their salvation.
Again, a lot of conflation here. Rom 5:6-10 is universal. It is all sinners, all the ungodly. This is confirmed by the statement in Rom 5:12 where the condemnation of death is passed to all men. Rom 5:18 states clearly that life was given to all men. This happens to align with I Cor 15:12-22, 52-54 as well.
None of the others are dealing with the atonement directly.
) Jn 17:9, 20 where before his sacrifice, Christ prayed only for those the Father had given
him, specifically excluding the "world" (the rest of mankind).
It is unconscionable that he would specifically exclude any for whom he intended to die.
typical proof texting. John 17:9 is directed at the disciples. If you really want to know what was given to Christ, Read Col 1:20. II Cor 5:18-19, Rom 3:24, Everything was given to Him. There is not a single exception in this created order that He did not redeem. The text of John 6:39 aligns with this because everything that was given to Him will be raised in the last day. He lost none. Kinda impossible to lose anyone when He assumed our mortal human nature for the express purpose of raising it to life by defeating death. The death of all men, Heb 2:9, Heb 2:14-17 tells us very plainly that He was Incarnated so that He could defeat death, which is the power of Satan. II Tim 1:10 says the same thing, as does I John 3:8.
And all in perfect agreement with the promise that all who come to Christ in faith will find mercy
(Jn 6:35, 47-51, 54-57; Ro 1:16, 10:8-13).
Your texts have nothing to do with atonment itself. These also are showing the purpose of the atonement. Why Christ needed to defeat death and sin. Actually two texts summarize it very well. I John 2:2. He atoned for the sin of the world. Heb 2:9 He tasted death for all men. He is indeed the Savior of the world, John 4:42, I John 4:14.
There is no limitation of Christ in His atoning work. It is impossible that it could be limited. It is universal from beginning to end because death and sin are universal. He did not save individuals from the Cross. His work reverses the fall, so that God and man can be reunited in an eternal union. God is now calling all men to repentance. He could never do this if only some sin and only death for some were atoned.
The atonement is between God and Christ, which was for the purpose of God uniting with man. This was God's original intent in creating man. Christ recreated the world, so that this union could go forward. Death precluded man ever having an eternal existence.
So, by accident, only one text, though you incorrectly labled the text, deal with the atonement itself. They all deal with the why, the purpose of the atonement.
One could also summarize it in two words, sacrifice and life.