How can we be assured of Eternal Life? God tells us through His Word, The Bible. We as Christians know that the Bible is the Word of God, and therefore completly accurate and trustworthy. We as believers can, by faith put our trust in it's teachings.
"For the wages of sin is death, but the free gift of God is eternal life in Christ Jesus our Lord." Romans 6:23
"That if you confess with your mouth Jesus as Lord, and believe in your heart that God raised Him from the dead, you will be saved." Romans 10:9
"Truly, truly I say to you, he who hears My word, and believes Him who sent Me, has eternal life, and does not come into judgment, but has passed out of death into life." John 5:24
"For God so loved the world, that He gave His only begotten Son, that whoever believes in Him shall not perish, but have eternal life." John 3:16
"My sheep hear My voice, and I know them, and they follow me; and I give eternal life to them, and they will never perish; and no one will snatch them out of My hand." John 10:27-28
"And the testimony is this, that God has given us eternal life, and this life is in His Son." 1 John 5:11
"These things I have written to you who believe in the name of the Son of God, so that you may know that you have eternal life." 1 John 5:13
"But these have been written so that you may believe that Jesus is the Christ, the Son of God; and that believing you may have life in His name. John 20:31
"But when the kindness of God our Savior and His love for mankind appeared, He saved us, not on the basis of deeds which we have done in righteousness, but according to His mercy, by the washing of regeneration and renewing by the Holy Spirit, whom He poured out upon us richly through Jesus Christ our Savior, so that being justified by His grace we would be made heirs according to the hope of eternal life." Titus 3:4-7
"It is a trustworthy statement, deserving full acceptance, that Christ Jesus came into the world to save sinners, among whom I am foremost of all. Yet for this reason I found mercy, so that in me as the foremost, Jesus christ might demonstrate His perfect patience as an example for those who would believe in Him for eternal life. 1 Timothy 1:15-17