I recently converted to Christianity and so did my girlfriend. We both have been in a very sexual relationship before we found God. We have been trying hard not to do anything sexual, but we messed up once, felt horrible and asked for forgiveness. We're getting baptized soon, and we're both excited. We agreed to not do anything sexual until marriage, but its been really hard. We have been talking and we want to have sex once then completely call off sex. My question is: Will God forgive us or will He not forgive us because we did it on purpose?
There are several things in your note that causes me to question the teachings you have been receiving. A person becomes a Christian when he is baptized. While you are in the process of becoming a Christian before baptism, you are not a part of Christ until you join with him in his death.
"For you are all sons of God through faith in Christ Jesus. For as many of you as were baptized into Christ have put on Christ" (Galatians 3:26-27).
"Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life" (Romans 6:3-4).
This is why you see an urgency in people when they learn the gospel. They don't wait around for a convenient time to be baptized, they are baptized immediately.
"Then those who gladly received his word were baptized; and that day about three thousand souls were added to them" (Acts 2:40).
"Then they spoke the word of the Lord to him and to all who were in his house. And he took them the same hour of the night and washed their stripes. And immediately he and all his family were baptized" (Acts 16:32-33).
"And now why are you waiting? Arise and be baptized, and wash away your sins, calling on the name of the Lord" (Acts 22:16).
"For He says: "In an acceptable time I have heard you, And in the day of salvation I have helped you." Behold, now is the accepted time; behold, now is the day of salvation" (II Corinthians 6:2).
One of the basis of becoming a Christian is a conviction that sin is wrong and desire to change your behavior regarding sin. This is called "repentance," a turning around in the direction your life is going. "... that they should repent, turn to God, and do works befitting repentance" (Acts 26:20). Thus, when the Jews realized that they had crucified their Messiah and wanted to know what to do about it, Peter told them, "Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit" (Acts 2:38).
The fact that you want to sin one more time tells me that you aren't fully convinced about the seriousness of sin. The problem is that carrying such an attitude will likely mean that you'll find other reasons to do it one more time and tell yourself you can repent of it later. "What shall we say then? Shall we continue in sin that grace may abound? Certainly not! How shall we who died to sin live any longer in it? Or do you not know that as many of us as were baptized into Christ Jesus were baptized into His death? Therefore we were buried with Him through baptism into death, that just as Christ was raised from the dead by the glory of the Father, even so we also should walk in newness of life. For if we have been united together in the likeness of His death, certainly we also shall be in the likeness of His resurrection, knowing this, that our old man was crucified with Him, that the body of sin might be done away with, that we should no longer be slaves of sin. For he who has died has been freed from sin" (Romans 6:1-7). Don't get me wrong, I'm glad you are getting baptized, but I want it to be for real and not a pretense. You need to understand you are giving your life over to Christ (Galatians 2:20) and along with that you are entering a war against sin (Ephesians 6:10-17).
It isn't that God can't forgive a purposeful sin, the problem is whether you are able to truly repent of sin. "For godly sorrow produces repentance leading to salvation, not to be regretted; but the sorrow of the world produces death. For observe this very thing, that you sorrowed in a godly manner: What diligence it produced in you, what clearing of yourselves, what indignation, what fear, what vehement desire, what zeal, what vindication! In all things you proved yourselves to be clear in this matter" (II Corinthians 7:10-11). Repentance is seen in the radical change in your behavior.