We can know for sure that we are saved. Jesus promised it. The Holy Spirit testifies about it. God's word affirms it if you received and trusted Jesus as Lord and Savior.
- If you believed (trusted) Jesus Christ and have been born again John 3:3, you have eternal life. John 3:16
- Jesus promised that we became children of God when we received Him. John 1:12
- The Holy Spirit assures us that we are God's children. Romans 8:16
- We crossed over from death to life when we believed Jesus as our Savior. John5:24
- The Bible affirms that we have life when we have the Son of God (Jesus Christ). 1 John5:11-13
- Our salvation does not depend on our ability to follow God but on God's ability to keep us from falling Jude 24 and complete what God has started in us. Philippians 1:6.
Does it need a "special feeling" or emotion to be sure about it? No, the basis of our assurance is the promise of God in His word and the testimony of the Holy Spirit in us, not our emotion. Our emotion, which changes depending on circumstances, is not a reliable gauge of our assurance of salvation. If one sincerely received Christ and trusted Him as Savior and Lord, regardless of emotional state, is saved.
Am I still saved after falling into sin? God word declares that "If we claim to have fellowship with him and yet walk in the darkness, we lie and do not live out the truth 1 John 1:6. Also, "If we claim to be without sin, we deceive ourselves and the truth is not in us. If we confess our sins, he is faithful and just and will forgive us our sins and purify us from all unrighteousness"1 John 1:8-9. The first passage tells us that a person is not saved, in the first place, if he deliberately lives in sin. He is not a true believer. A person who genuinely received Jesus Christ does not continue in sin. But the next passage teaches us that we cannot claim to be sinless even if we are saved. Yes, Christians also make mistakes and, sometimes, commit grievous sin but because he has a seal of redemption which is the Holy Spirit Ephesians 1:13-14 which he received at the moment he received Christ, he is still saved. Nothing and no one could take us away from our Savior John 10:28-30. When we sin, God is not happy and we need to confess our sins (to God) to receive forgiveness and cleansing of our unrighteousness.
I backslid, do I need to received Christ again to be saved (again)? No. You do not need to receive Christ again to be "saved again". There is no such thing as "saved again". If a person received Christ, then he was born again, became a child of God and therefore saved eternally John 3:15-16. If he backslides, he will lose the joy of God's salvation but not the salvation itself. He would lose the joy of Christian fellowship and the intimacy of his relationship with the Lord. His faith could become weak and his lifestyle could become like of those who do not know Christ. The Holy Spirit is grieved when a child of God lives this way but will continue to convict the backslider until he returns to God. The parable of the prodigal son Luke 15:11-24 is good biblical lesson that teaches that our God waits for the backsliding child to awaken, confess his sins and return. God welcomes that child back to Himself.
Is the assurance a license to do anything I want (including sinning), since I will not lose my salvation? The assurance of salvation is not a license to sin. Rather, it is a great motivation to be like Jesus Christ in His character and righteousness. No true Christian will use his salvation to justify unrighteous and immoral life. Romans 6:1 1John 3:7-9.