I have one question for you because I'm really confuse what to decide. Please help me. My question is that my girlfriend and I plan to marry next year. We are both Christian, but we have already been having sex. I never had sex with another women and she has only had sex with me. Is it a sin to marry after having sex? What should we do so that God will forgive us? Please give me some advice.
"Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived. Neither fornicators, nor idolaters, nor adulterers, nor homosexuals, nor sodomites, nor thieves, nor covetous, nor drunkards, nor revilers, nor extortioners will inherit the kingdom of God" (I Corinthians 6:9-10).
Among the many problems in the church at Corinth, fornication was prevalent. In this passage Paul is getting across the point that being a Christian does not protect you from the consequences of the sins you might commit. Unrepented sin keeps people out of heaven, regardless of who commits the sin. Therefore, let us first note that you two are involved in sin -- sin that will keep you out of heaven if you continue.
It isn't a sin to get married. The sin is in your having sex without being married. "But I say to the unmarried and to the widows: It is good for them if they remain even as I am; but if they cannot exercise self-control, let them marry. For it is better to marry than to burn with passion" (I Corinthians 7:8-9). Intentions of getting married doesn't excuse the fact that you are currently sinning by committing fornication. Getting married will stop the fornication because you will then be married, but it doesn't mean your sins will be forgiven. A thief can stop stealing, but if he hasn't changed his mind about his sin along with the change in his behavior, he has not repented. For example, thief in jail will stop stealing, but that doesn't mean he has changed his mind about his sin. Nor is sorrow over past deeds repentance. See Being Made New for a detailed discussion of this.
Therefore, it starts with both of you deciding that having sex while not married is wrong, if for no other reason than because God said so. (See Why Sex Outside of Marriage is Wrong.) From there the two of you need to decide how best to reduce the temptation. If you can't marry until next year, then you are going to have to continue your relationship while no longer sinning. Few couples actually survive that because they soon learn that all they had in common was the sex. Without the sex, they actually have to get to know the other person. You can decide, instead, to move your wedding day up and get married so that you don't have to deal with temptation.
Forgiveness depends on doing as God requires. You cannot live for yourselves and expect to live for God. For Christians that means truly repenting of your sins (II Corinthians 7:10-11) and admitting to God that you were wrong (I John 1:9).