Whether most Christians think about it or not, their entire belief system is based on the premise that God is faithful. God's faithfulness is what makes it reasonable for us to trust in Him and devote our lives to doing His will. We know that if we keep our covenant with Him, He will not disappoint us.
If this were not true, if God were arbitrary and capricious rather than faithful, we would have no reason to seek Him at all. The day of judgment would be a spiritual lottery, in which one person would be unpredictably rewarded while another is unpredictably punished. At that point, it makes more sense for us to do what is right in our own eyes and take our chances.
As the above example implies, God's faithfulness extends both to those who serve Him and to those who do not. We want God to be faithful so that we can rely on His promises. However, His faithfulness also must mean that we can rely on His warnings too.
Like an inconsistent parent, a God who makes threats and doesn't carry them out can't be trusted (and also won't produce good behavior). If we want to put our trust in God's statement that whoever believes in Jesus will not perish, but have eternal life, we must put equal trust in His statement that Jesus will return to inflict vengeance on those who do not obey the gospel. In fact, the Scriptures arguably provide more evidence that God will be faithful to His warnings than they do that God will be faithful to His promises.
This is generally relevant, but its implications are most significant in matters pertaining to salvation. God is merciful to those who repent and submit, but He remains opposed to the proud and hard-hearted. Even the intercession of the righteous (Abraham for Sodom, Moses at Sinai) can at best postpone the day of reckoning. What's more, if we want His favor, we must seek it in the way He specifies. He extends His mercy on His terms, not ours.
For most, the sticking point here is the command to be baptized. Because baptism itself is such a simple act (how many people anywhere have never been immersed in water at some point?), it's evident that the godly intent with which we approach the water is what distinguishes baptism from a bath. Conversely, if the validity of baptism is not determined by intent, we are left with the strange conclusion that it is possible to be saved by accident. Is God's wrath ever averted by unintentional obedience?
The Bible doesn't leave us in much doubt about what our intent needs to be. Not every text about baptism describes the intent of the baptizee (as opposed to the purpose or effect of baptism), but those that do point in the same direction. Here's my list of every passage that provides or implies the baptizee's intent. Others' lists may vary by a passage or two, but that won't change the conclusion.
- To enter the kingdom of God (John 3:5)
- To have sins forgiven (Acts 2:38)
- To wash away sins (Acts 22:16)
- To rise to walk in newness of life (Romans 6:4)
- To become a son of God (Galatians 3:26-27)
- To be raised with Christ (Colossians 2:12)
- To appeal to God for a good conscience (I Peter 3:21)
This is a tightly focused list. We don't see generic texts that say, "Get baptized because you want to obey God," or any other such thing. Instead, people in the first century who went down into the water did so because they wanted to be spiritually transformed, to leave behind their old sinful selves and follow Jesus. Anybody who is baptized for any other reason isn't taking their marching orders from the word of God.
God saves people who seek Him according to His word. He is faithful. He does not save those who do not seek Him according to His word (and I'm not aware of a single counterexample anywhere in the Bible). Again, this is because He is faithful. If we are confident in His promise of salvation, we have every bit as much reason to be confident in His promise of judgment.
Either both of those promises are reliable, or neither one is.