Because you reject the revelation how Adam died in the day he ate from the tree, you cannot understand "death is separation from God". Physical death is a consequence of spiritual death.
God is Holy, what is dead is unholy. Adam cannot abide in God, God's Holy Being won't permit it.
To illustrate. Adam is a finite being of fire. God is infinite ocean of pure water. When Adam tries to enter God, he gets snuffed out.
Only in Christ is there atonement with God. Only in Christ can Adam pass from being fire, into the water.
NKJ John 5:24 "Most assuredly, I say to you, he who hears My word and believes in Him who sent Me has everlasting life, and shall not come into judgment, but has passed from death into life. (Jn. 5:24 NKJ)