There are many things which none of us can know--not even the apostle Paul. Yet even with his realistic modesty, there are certain key truths which Paul could affirm with certainty, and so can we on the same grounds as he.
One essential thing each of us should know first of all is this: "I know that in me (that is, in my flesh,) dwelleth no good thing" (Romans 7:18). A person needs to know that he is a lost and hopeless sinner before he will ever really come to Christ for salvation.
Once a lost sinner does receive Christ as Savior, however, he then should be able to declare with Paul the certainty of his own salvation. "For I know whom I have believed, and am persuaded that he is able to keep that which I have committed unto him against that day" (2 Timothy 1:12).
The Christian life, once begun, is not necessarily easy. With Paul, in fact, it involved "labours more abundant, in stripes above measure, in prisons more frequent, . . . In weariness and painfulness, in watchings often, in hunger and thirst, in fastings often, in cold and nakedness" (2 Corinthians 11:23, 27). Yet he could say with confidence, "We know that all things work together for good to them that love God, to them who are the called according to his purpose" (Romans 8:28).
Because of such an assurance, he could also say, "I have learned, in whatsoever state I am, therewith to be content. I know both how to be abased, and I know how to abound" (Philippians 4:11-12). Whatever life might bring, it could never shake his certainty of the life to come. "For we know that if our earthly house of this tabernacle were dissolved, we have a building of God, an house not made with hands, eternal in the heavens" (2 Corinthians 5:1). HMM
http://www.icr.org/article/7124/