Once a person truly turns to God in repentance through faith in Christ Jesus as Lord and Savior, they are saved; they cannot lose their salvation. (Ephesians 1:13-14) As God says: "Therefore, there is now no condemnation for those who are in Christ Jesus," (Romans 8:1 NIV) All of the guilt, and penalty, of their sin is removed (once and for all). God chooses to remember their sin no more, as to their salvation. (Hebrews 8:12)

When a child of God (a Christian) gives into temptation, and falls into sin, however, what they need to do is turn away from it (repent of it) and seek God’s forgiveness. When they do, God has said that He will forgive them of their sin and the shed blood of Christ Jesus on the cross of Calvary (when sin was judged) continues to purify them from all unrighteousness. (1 John 1:9; Romans 8:1)

As to your question on Christian baptism, baptism does not save one. It is not essential to salvation in that it is required in order for one to be saved.

In Christian baptism, the one being baptized is testifying to the world that they were in Christ Jesus when sin was judged on the cross of Calvary, that they died with Him, were buried with Him, and have arisen to a new life in Him. (Romans 6:3-4) They are also testifying that they are no longer an unbeliever, but, rather, a believer in Christ Jesus as Lord and Savior. (Acts 2:38)

Although baptism is not essential to salvation, God has asked His children to be baptized as a testimony of what has taken place in their lives. It is not to be repeated over and over, however, many who were baptized before they truly turned to God in repentance through faith in Christ Jesus as Lord and Savior choose to be baptized again (when it truly means something).