But what if I mess up, big time? What if, somewhere along the way for whatever reason, I yield to the devil's temptations and I'm taken in by the seeming allure of sin? What if I become spiritually lazy? What if I stop living by the teachings of Jesus? What if I become unfaithful?! How can I come back to the Lord?
First, I need to remember that sin is deadly, eternally so (2 Thessalonians 1:7-9). The Lord did not walk away from me, I walked away from Him by turning back to sin and away from His teachings: "...your iniquities have separated you from your God; and your sins have hidden His face from you, so that He will not hear" (Isaiah 59:2). I need forgiveness again.
But while I again stand lost in sin, just as I did before I first came to my Lord, I'm nonetheless in a different relationship with Him now than I was then. Before I initially came to believe in Jesus as my Lord and Savior, and repented and was baptized into Him for the remission of my sins, I was an alien sinner. I was outside of Christ. I was not a part of His body, the church.
When I was baptized, I came into Christ (Galatians 3:27). He added me to His body (Acts 2:47; 1 Corinthians 12:13). In turning back to sin I became unfaithful to Christ. In order to come back to Him I must again repent of my sins. But I don't need to be baptized again, because true baptism is one time for all time (Ephesians 4:4-5). But I do need forgiveness. Just as Peter instructed Simon, when he had sinned after becoming a Christian, I need to humbly pray to my Heavenly Father for His forgiveness again (Acts 8:20-22).
As I humble myself before God and seek His forgiveness through prayer, Jesus will be there in Heaven pleading my case before His Father's throne. The blood that my Lord shed on the cross is still effective to cleanse me again of the guilt of my new sins (1 John 2:1-2). As the prodigal son's father lovingly accepted him back upon his repentance and return home, I can rest assured that my Heavenly Father will forgive me and accept me back. I will have come home, back to my Lord...