What assures you that what you believe is right when you begin to have doubts?
Questioning our faith is something that we should all do. Before giving the status of truth to something , it is a good idea to verify its veracity. This is an excellent way to build a stronger foundation to our beliefs.
Our Christian faith rests heavily on what Jesus said, what he did, and what happened. A careful analysis of the gospel testimonies and the letters of the NT shows that these can be trusted. Further, historical evidences reveal that Jesus really existed and that he was crucified under Ponce Pilate and reported resurrected.
The resurrection is obviously the most fundamental reason to believe in God. In fact, if Christ is not resurrected then there is no reason to believe in any God. Although there are no direct evidences of Jesus resurrection, there are certainly indirect evidences. For example, the apostles abandoned Jesus when he was arrested and Peter even said that he did not know that man. However, after the resurrection, they all proclaimed that Jesus is the Messiah, the Lord and is risen from the dead. They had nothing to win from such claims but insults, persecutions and violent deaths. Similarly, Paul himself said that he persecuted the church because he wanted to get rid of that faith. However, after he saw the resurrected Lord, he then proclaimed the faith he wanted to destroy. Such changes in behavior clearly point to a radical event in their live, namely, the resurrection of the Lord.
The best way to know if God exist is obviously to experience his presence. May the Holy Spirit dwells in your heart forever ...