The following verses directly relate to the subject of eternal assurance. Emphasis (italics, bold or comments in parenthese) have been added because I feel that they vividly point to the purpose of this study. Before you start your study, pray. Ask for the Holy Spirit to speak directly to your doubt. The greatest starting point for me is to read the "summary" that tells why the Holy Scriptures was written. 1st John 5:13 says, "I write these things to you who believe in the name of the Son of God so that you may know that you have eternal life."
Self look-up and study verses:
Romans Chapter 3
Romans 8: 31-39
1st Corinthians 15: 19-26
There are several topics that directly relate to the subject of eternal assurance. Some are:
Eph 3:12 In him and through faith in him we may approach God with freedom and confidence. (NIV)
2 Tim 1:12 12 That is why I am suffering as I am. Yet I am not ashamed, because I know whom I have believed, and am convinced that he is able to guard what I have entrusted (my eternal salvation) to him for that day. (NIV)
Heb 10:22 22 let us draw near to God with a sincere heart in full assurance of faith, having our hearts sprinkled to cleanse us from a guilty conscience and having our bodies washed with pure water. (NIV)
Heb 6:17-19
17 Because God wanted to make the unchanging nature
of his purpose very clear to the heirs of what was promised, he
confirmed it with an oath.
18 God did this so that, by two
unchangeable things in which it is impossible for God to lie, we who
have fled to take hold of the hope offered to us may be greatly
encouraged.
19 We have this hope as an anchor for the soul,
firm and secure.
(NIV)
I Jn 3:18-20
18 Dear children, let us not love with words or
tongue but with actions and in truth.
19 This then is how we
know that we belong to the truth, and how we set our hearts at rest
in his presence
20 whenever our hearts condemn us. For
God is greater than our hearts, and he knows everything.
(NIV)
I Jn 4:15-17
15 If anyone acknowledges that Jesus is the Son of
God, God lives in him and he in God.
16 And so we know and
rely on the love God has for us. God is love. Whoever lives in love
lives in God, and God in him.
17 In this way, love is made
complete among us so that we will have confidence on the day of
judgment, because in this world we are like him.
(NIV)
Rom 8: 38-39
38 For I am convinced that (nothing)
neither death nor life, neither angels nor demons, neither the
present nor the future, nor any powers,
39 neither height nor
depth, nor anything else in all creation, will be able to separate
us from the love of God that is in Christ Jesus our Lord.
(NIV)
1 Cor 6:14-15
14 By his power God raised the Lord from the
dead, and he will raise us also.
15 Do you not know that
your bodies are members of Christ himself?
(NIV)
Eph 5:29-30
29 After all, no one ever hated his own body, but
he feeds and cares for it, just as Christ does the church--
30 for
we are members of his body.
(NIV)
I Jn 4:13-15
13 We know that we live in him and he in us,
because he has given us of his Spirit.
14 And we have seen and
testify that the Father has sent his Son to be the Savior of the
world.
15 If anyone acknowledges that Jesus is the Son of God, God
lives in him and he in God.
16 And so we know and rely on the love
God has for us. God is love. Whoever lives in love lives in God, and
God in him.
17 In this way, love is made complete among us so
that we will have confidence on the day of judgment, because in
this world we are like him.
(NIV)
Rom 5:1
1 Therefore, since we have been justified through
faith, we have peace with God through our Lord Jesus
Christ,
(NIV)
Phil 1:4-6
4 In all my prayers for all of you, I always pray
with joy
5 because of your partnership in the gospel from the
first day until now,
6 being confident of this, that he who
began a good work in you will carry it on to completion until the
day of Christ Jesus.
(NIV)
Ps 42:11
11 Why are you downcast, O my soul? Why so disturbed
within me? Put your hope in God, for I will yet praise him, my
Savior and my God.
(NIV)
Note: NIV stands for The Holy Bible, New International Version