This question is answered in our tract Assurance of Salvation?, which suggests the following reply:
As the Bible says, I am already saved (Rom. 8:24, Eph. 2:5-8), but I’m also being saved (1 Cor. 1:8, 2 Cor. 2:15, Phil. 2:12), and I have the hope that I will be saved (Rom. 5:9-10, 1 Cor. 3:12-15). Like the apostle Paul, I am working out my salvation in fear and trembling (Phil. 2:12), with hopeful confidence in the promises of Christ (Rom. 5:2, 2 Tim. 2:11-13).