So in essence, noone can ever be sure they are of the elect?
From the responses about that very thing, Paul says this. Verse 16 is something you feel and know about yourself and God, an internalized type idea that you know in your knower, it goes deep into your being this bearing witness from His Spirit to our spirit that we are the children of God. If you dont have this inner witness in your spirit, you need to ask Him with prayers and supplication to reveal this to you by His Spirit. In Acts 1 before the Spirit was given, poured out on them, the disciples spent time praying all of them together waiting on God
Romans 8
4 For as many as are led by the Spirit of God, these are sons of God.
15 For you did not receive the spirit of bondage again to fear, but you received the Spirit of adoption by whom we cry out, “Abba, Father.”
16 The Spirit Himself bears witness with our spirit that we are children of God,
17 and if children, then heirs—heirs of God and joint heirs with Christ, if indeed we suffer with Him, that we may also be glorified together.
Acts 1
12 Then they returned to Jerusalem from the mount called Olivet, which is near Jerusalem, a Sabbath day’s journey. 13 And when they had entered, they went up into the upper room where they were staying: Peter, James, John, and Andrew; Philip and Thomas; Bartholomew and Matthew; James the son of Alphaeus and Simon the Zealot; and Judas the son of James.
14 These all continued with one accord in prayer and supplication, with the women and Mary the mother of Jesus, and with His brothers.