You, however, are controlled not by the sinful nature but by the Spirit, if the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, he does not belong to Christ.
Here, we see the first function of the Holy Spirit and that is the fact that you can know that you are saved if you have the Spirit. There are many people that call themselves Christians but, if you ask them, they will tell you that they are not really sure of their salvation. That doubt is not of God but is a scheme of the devil. Our Father does not want us walking around unsure about His love for us. It is really very simple: If you have the Spirit, you are a child of God. If you don't, then, you are not. Got the Spirit?