I have never really understood what people mean by born again...
They mostly seem to reject theosis, so I'm confused
Born again is tied to believing in Christ's death and resurrection for our salvation and results in being given a new nature desirous of God, it is freely given upon faith in the death and resurrection of Jesus for our sins. Here are a few passages that may help.
2 Corinthians 5:16-17 (NASB) Therefore from now on we recognize no one according to the flesh; even though we have known Christ according to the flesh, yet now we know
Him in this way no longer. Therefore if anyone is in Christ,
he is a new creature; the old things passed away; behold, new things have come.
John 3:3-7 (NASB) Jesus answered and said to him, "Truly, truly, I say to you, unless one is born again he cannot see the kingdom of God." Nicodemus
*said to Him, "How can a man be born when he is old? He cannot enter a second time into his mother's womb and be born, can he?" Jesus answered, "Truly, truly, I say to you, unless one is born of water and the Spirit he cannot enter into the kingdom of God. "That which is born of the flesh is flesh, and that which is born of the Spirit is spirit. "Do not be amazed that I said to you, 'You must be born again.'
Here is where it ties into His death and resurrection...
John 3:14-15 (NASB) "As Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up; so that whoever believes will in Him have eternal life.
Here is where it is promised long ago...
Jeremiah 31:33-34 (NASB) "But this is the covenant which I will make with the house of Israel after those days," declares the LORD, "I will put My law within them and on their heart I will write it; and I will be their God, and they shall be My people. "They will not teach again, each man his neighbor and each man his brother, saying, 'Know the LORD,' for they will all know Me, from the least of them to the greatest of them," declares the LORD, "for I will forgive their iniquity, and their sin I will remember no more."
Misc...
1 Peter 1:23 (KJV) Being born again, not of corruptible seed, but of incorruptible, by the word of God, which liveth and abideth for ever.