Where Assurance Comes From
In his article "Where Can I Find Assurance?" Tullian Tchividjian explains why our assurance of salvation must come from outside of us. We cannot rely on internal feelings of guilt or uncertainty to sway us from the absolute truth of God's declarations to us. And we can't rely on feelings of certainty or peace to assuage our very real guilt before God.
Our feelings must be informed by God's view of us. What he sees is what really is. And God reveals our true reality in the Bible. His Word is true and certain no matter how our thoughts or feelings may vary.
Since our assurance is grounded in the absolute truth of God's Revelation, we see that it is in fact God's own character that assures us. His nature as Absolute Truth is the reason that we believe what at times does not feel or seem to be true.
Jesus Christ claims the authority of that very nature for himself when he says that he is "The Way, The Truth, and The Life." So to believe in Jesus is to believe in God himself. What Jesus has done for us, by living a perfect life, dying for our sins, and destroying death through his resurrection, is what brings us true assurance of salvation.
That's why when the apostle Paul writes to the Roman church about what they've experienced in salvation he doesn't speak to how they feel. Rather, in Romans 5, he describes their condition before God as based explicitly on the work of Jesus Christ on their behalf. They have recieved the Holy Spirit as a sign of God's love, and they know of God's love because of Christ's death.
From beginning to end the entire process of salvation is enacted by God. It's no wonder then that when we are unsure of our own place within that salvation that we must turn our gaze away from ourselves and remember just who it is that has declared us righteous before God.