How do you know you possess salvation? "Believe in your heart that God raised Him from the dead you will be saved" (Ro 10:9). "The one who believes in the Son of God has the testimony in himself" (1Jn 5:10) for "he who has the Son has the life" (:12). "Whoever believes in Him will not be disappointed" (Ro 10:11) for "as many as received Him, to them He gave the right to become children of God, even to those who believe in His name" (Jn 1:12). "The Spirit Himself testifies with our spirit that we are children of God" (Ro 8:16). "We have obtained an inheritance" (Eph 1:11) and are "heirs of God and fellow heirs with Christ" (Ro 8:17) "with a view of the redemption of God's own possession" (Eph 1:14). He "gave us the Spirit in our hearts" (2Co 1:22) "as a pledge of our inheritance" (Eph 1:14).
How do you experientially know that you are a "new creation in Christ" (2Co 5:17). "By grace you have been saved through faith; and that it is not of yourselves, it is the gift of God" (Eph 2:8). It is "bestowed on us" (1:6) and "He has "blessed us with every spiritual blessing" (:3). "The love of God was manifested in us" (1Jn 4:9) and "God is for us" (Ro 8:31) and "intercedes for us" (:34). It is "an inheritance which is imperishable and undefiled and will not fade away, reserved in heaven for you" (1Pe 1:4). "He is able to guard" (2Ti 1:12) and "perfect it until the day of Christ Jesus" (Php 1:6). You are "protected by the power of God through faith" (1Pe 1:5).
Is there a guaranty that what you believe in will be fulfilled? Abraham was "fully assured that what God had promised, He was able also to perform" (Ro 4:21). "The one who endures to the end will be saved" (Mk 13:13) "if we hold fast . . . until the end" (Heb 3:14). Some have commented that if there wasn't any ongoing responsibility of the believer to continue in faith then maybe God would rapture each one at the point of salvation. Calvinists believe that those "He foreknew, He also predestined" (Ro 8:29) takes away the threat of backsliding. But why would Peter warn "brethren, be all the more diligent to make certain about His calling and choosing you" (2Pe 1:10). He answers "as long as you practice these things, you will never stumble" (:10). There are many scriptures you can rely on but they must experientially become a part of you. Therefore make sure you are "building yourselves up on your most holy faith, praying in the Holy Spirit" (Jude 20). It requires your effort to "keep yourselves in the love of God (:21). Furthermore, you must be "waiting anxiously for the mercy of our Lord" (:21). Anxiously means with anticipation and diligence.