Eternal Security is a guarantee of God to His redeemed, denied by the flesh because it (the doctrine) puts all confidence in Christ and the Bible, whereas the flesh wants to brag on itself and how Christ needs help - both in the converting and in the perseverance of the saints.
Study up on the cultural context of being adopted into God’s family (under Roman law, adopted children could not be kicked out of the family), of having the Lord build a room onto the Father’s house (John 14), and of Christ being the ruler of all creation. He needs our help to hold onto the ones He chose in the first place? When the primary goal is to glorify God, He will not fail. And the primary goal the Lord has in redeeming sinful man is His glory. Our salvation is to His glory - its secondary attribute is to our eternal benefit.
‘tis a pitiful god who can’t keep what he declares to be his.
'Tis a weak "god" that denies free will to his creation
The saved are those who persevere in faith and obedience.