"And we desire each one of you to show the same earnestness to have the full assurance of hope until the end, so that you may not be sluggish, but imitators of those who through faith and patience inherit the promises." (Hebrews 6:11-12 ESV)
"I write these things to you who believe in the name of the Son of God that you may know that you have eternal life." (1 John 5:13 ESV)"Assurance" is another way of saying confidence in God. There are two dimensions or parts to assurance: having confidence that God truly is who He says He is and will do all He has promised; and having confidence that you are indeed forgiven by God for all your sins and accepted as His child (being "saved").1 The second part of assurance is believing that you have personally received salvation and forgiveness and that you are now accepted by God. If you don't have the first part secure, trusting God to be who the Scriptures say He is and trusting that He will do what He has promised, the second part doesn't give you much confidence. Being sure you are God's child is an uncertain comfort if you don't believe God is dependable. But if you get the first part straight and still doubt that God specifically loves and accepts you, then you will be doubtful and insecure about whether you personally will benefit from His dependability and faithfulness. So it is crucial to work on being confident that God loves you personally and fully intends to fulfill all His promises in your life.
Spurgeon nails the most important part of this: "[W]e shall never find comfort or assurance by looking within. But the Holy Spirit turns our eyes entirely away from self ... We shall never find happiness by looking at our prayers, our doings, or our feelings; it is what Jesus is, not what we are, that gives rest to the soul. If we would at once overcome Satan and have peace with God, it must be by ‘fixing our eyes on Jesus.’" (Morning and Evening, p. 360). Assurance is found in seeking to know God in the fullest way possible, largely through examining the Scriptures, which display the best and most complete picture of Him we can have, and through seeking Him directly in prayer. And here it is also vital to remember that Jesus Christ is the way God has revealed Himself to us, and it is through Jesus that we come to know God. "He is the radiance of the glory of God and the exact imprint of his nature, and he upholds the universe by the word of his power." (Heb. 1:3). "No one has ever seen God; the only God [Jesus], who is at the Father's side, he has made him known. (John 1:18). If you would know God, fix your eyes on Jesus.
The more you know God, the more you see He is trustworthy. And the more you know God, the more you discover whether what you see is lovely and appealing and precious. One of the best confirmations that you are accepted and loved by God is that you love what you see of Him. And because our Lord loves us, He does not want us to neglect that source of joy and comfort.
The seventy-two returned with joy, saying, “Lord, even the demons are subject to us in your name!” And he said to them, “I saw Satan fall like lightning from heaven. Behold, I have given you authority to tread on serpents and scorpions, and over all the power of the enemy, and nothing shall hurt you. Nevertheless, do not rejoice in this, that the spirits are subject to you, but rejoice that your names are written in heaven.” (Luke 10:17-20)
1 For those interested, here is the definition from Louis Berkhof's Systematic Theology: "There is a twofold assurance, namely, (1) The objective assurance of faith, which is 'the certain and undoubting conviction that Christ is all He professes to be, and will do all He promises.' … (2) The subjective assurance of faith, or the assurance of grace and salvation, which consists in a sense of security and safety, rising in many instances to the height of an 'assured conviction that the individual believer has had his sins pardoned and his soul saved.'" (pp. 562-63).