Question: "Is it necessary to fully understand the Gospel to go to heaven? Is it enough to believe it, even if we do not fully understand it?"
Answer: It is important to recognize that no one fully understands all of the depth and nuances of the riches found in the Bible. The Bible is God’s revelation of Himself to mankind, and since God is infinite, the knowledge of Him is also infinite. But the mind of man is finite and is therefore incapable of fully understanding the depth of Truth. Romans 11:32-34 expresses it thus: “Oh, the depth of the riches of the wisdom and knowledge of God! How unsearchable his judgments, and his paths beyond tracing out! 'Who has known the mind of the Lord? Or who has been his counselor?'" God reminds us through the prophet Isaiah, “As the heavens are higher than the earth, so are my ways higher than your ways and my thoughts than your thoughts” (Isaiah 55:9).
That said, however, man is certainly capable of understanding the basic doctrines of the Bible and the knowledge of God which He has made clear to us (Romans 1:19). He has provided in His Word all we need to know for salvation and godly living. God does not try to confuse us when it comes to any biblical topic, especially salvation; “For God is not the author of confusion, but of peace” (1 Corinthians 14:33 KJV). Likewise, He has made it simple to understand how we are saved. When asked what needed to be done in order to be saved, Paul and Silas made it very clear: "Believe on the Lord Jesus Christ, and you will be saved – you and your household" (Acts 16:31). In John 6:28-29, Jesus was asked, “What must we do to do the works God requires?” to which Jesus answered, “The work of God is this: to believe in the one He has sent.” Jesus also said, “I am the resurrection and the life. He who believes in me will live, even though he dies” (John 11:25). The Bible declares that it is faith that saves us, not understanding. But genuine faith will result in more and more understanding of God’s Word as we grow in Christ because the Author of Scripture—the Holy Spirit—dwells in our hearts, and He will lead us into all truth (John 16:13). Without Him, we are unable to understand spiritual truth (1 Corinthians 2:14). So the first requirement for understanding Scripture is a true saving faith in Christ.
One reason many give for not reading their Bible is that they “just don’t understand it.” This is sometimes due to the translation they’re reading, so it may be a good idea to look into other translations that might make reading and understanding the Bible easier. For instance, many people find the 17th century English of the King James Version (KJV) difficult to understand and might benefit from reading a more modern English translation like the New King James Version (NKJV), New International Version (NIV), or New American Standard Bible (NASB).
Discernment is extremely important when reading and studying Scripture, so before sitting down and studying the Bible, regardless of which translation we choose, it is always a good idea to pray and ask God to help us understand what we’re reading. He has promised to give wisdom to all who ask for it (James 1:5). Context is also very important when studying the Bible, so it's a good idea to study entire chapters and books as opposed to simply trying to make sense of random individual passages. Jesus told the Sadducees that they were “in error” because they “didn’t know the Scriptures or the power of God” (Matthew 22:29). The error of the Sadducees was that they relied on their own power to interpret Scripture, rather than the power of the One who wrote it.
The Bible makes it perfectly clear that “all Scripture is God-breathed and is useful for teaching, rebuking, correcting and training in righteousness, so that the man of God may be thoroughly equipped for every good work” (2 Timothy 3:16-17). Psalm 119:130 declares that “the unfolding of your words gives light; it gives understanding to the simple.” However, this understanding doesn’t come with periodic skimming through popular Bible verses; it comes with eagerness to study and examine the Scriptures every day as the Bereans did (Acts 17:11). Paul said that the Bereans were of more noble character than even the Thessalonians because of their eagerness to search the Scriptures.
All too often, people do not understand the Bible because they are reluctant to accept what it says: “Their ears are hard of hearing, and their eyes they have closed, lest they should see with their eyes and hear with their ears, lest they should understand with their heart and turn, so that I should heal them” (Matthew 13:15 NKJV). Regardless of our personal feelings, we are reminded to “be very careful, then, how you live—not as unwise but as wise, making the most of every opportunity, because the days are evil. Therefore do not be foolish, but understand what the Lord's will is” (Ephesians 5:15-17).
Lastly, to be as clear and specific as possible, John wrote, “For God did not send his Son into the world to condemn the world, but to save the world through Him. Whoever believes in Him is not condemned, but whoever does not believe stands condemned already because he has not believed in the name of God's one and only Son” (John 3:17-18). The common theme among the passages that refer to salvation is to “believe.” There may be passages that we simply don’t understand fully, but if our faith in Jesus as our personal Lord, Savior, and Redeemer is genuine, we will continue to gain more understanding as we read, study, and rely on the Holy Spirit to guide us into all truth.