Question: "Is there any sin that God will not forgive?"
For the born-again child of God, there is no unforgivable sin. All sin was forgiven at the cross. When Christ Jesus said, "It is finished" (John 19:30), that statement meant that the penalty for all sin was paid in full. The word translated "it is finished" is the Greek word “tetelestai.” That word was used in several ways. It was used to stamp "paid" upon a receipt, and it was also the stamp put on a criminal's charges once he had completed his sentence. A "tetelestai" was nailed to the door of his house proving that he had indeed paid in full for his crimes.
You can see the application to the Cross transaction between the Lord Jesus and God the Father. Jesus Christ completed the legal transaction and satisfied God's holy and righteous demand as the payment for the sin of "whosoever will." The Lord Jesus Christ became our sin sacrifice and “the Lamb of God who takes away the sin of the world" (John 1:29). When Christ was separated from God the Father for those three hours of supernatural darkness (Matthew 27:45), the deal was sealed. As we read in Luke, Jesus was reunited with the Father. "And when Jesus had cried with a loud voice, he said, Father, into thy hands I commend my spirit: and having said thus, he gave up the ghost" (Luke 23:46). Therefore, all sin was paid for once for all.
However, there is a condition upon God's forgiveness of sin. Man must come to God through the Lord Jesus Christ alone. "Jesus said to him, ’I am the way, the truth, and the life. No one comes to the Father except through Me’" (John 14:6). God's forgiveness is available to all who will come (John 3:16), but for those who will not believe upon the Lord Jesus Christ, there is no forgiveness or remission of sin (Acts 10:43). Therefore, the only sins God will not forgive in this age of grace are the sins of those who die without first placing their faith in Jesus Christ. By that I mean that a person goes through his life here on this earth and fails to avail himself of the provision that God has provided through the Lord Jesus Christ and goes out into eternity separated from God, and therefore unforgiven.
Born-again believers also sin, and when we do, we put ourselves outside of fellowship with the Lord. However, God has made a provision for that. The Holy Spirit that indwells every born-again believer convicts us and convinces us that we have sinned, and when that happens we have a choice to respond in the right way and renew our fellowship. Once a person is born again and has accepted Christ as his Savior and received remission of sin, there is no way he can lose his eternal life based upon his actions. We can lose our fellowship with God and the joy of our salvation, but that is something we can remedy through confession.
The first epistle of John is a letter written to born-again believers, and it has very practical information on how to walk in fellowship. "If we confess our sins, he is faithful and just to forgive us our sins, and to cleanse us from all unrighteousness" (1 John 1:9). This verse, when used correctly, becomes the way to restore our fellowship when we sin, and we will. "If we say that we have no sin, we deceive ourselves, and the truth is not in us" (1 John 1:8). Now, remember, this is a letter to born-again believers. God has no illusions about us and our capacity to sin, and we should not either.
The "if" at the beginning of both 1 John 1:8 and 9 is a third-class "if" in the Greek, and it means "maybe yes, maybe no." There is a condition here; if we "confess." This word in the Greek is "homologia," and it means to say the same thing or cite the case. "Homo" is "same," and "logia" is "word." It means we agree with God that we have sinned. But all sin was forgiven at the Cross and, as born-again believers, all of our sin has been forgiven. And because that is a judicial fact, we need to walk in light and in fellowship because that is our position in Christ Jesus. "But if we walk in the light as He is in the light, we have fellowship with one another, and the blood of Jesus Christ His Son cleanses us from all sin." (1 John 1:7). That does not give us carte blanche to continue sinning; rather, a born-again believer who is walking in the light and fellowship of God will be quick to use confession so that there remains a continual and clear fellowship with the Lord on a daily basis.