To determine whether or not water baptism is required for salvation, we must first look at what salvation entails. The most significant factor is that of the Holy Spirit's indwelling. According to Scripture having the Holy Spirit is the mark of whether or not a person is saved.
Romans 8:9 You, however, are controlled not by the sinful nature but by the Spirit, if the Spirit of God lives in you. And if anyone does not have the Spirit of Christ, he does not belong to Christ. 10 But if Christ is in you, your body is dead because of sin, yet your spirit is alive because of righteousness. 11 And if the Spirit of him who raised Jesus from the dead is living in you, he who raised Christ from the dead will also give life to your mortal bodies through his Spirit, who lives in you.
As we see in this passage, if the Spirit is not in you, you are not saved, but those who do have the Spirit are. Anyone without the Spirit simply does not belong to Christ, but everyone who has the Spirit will be raised again with Christ (which is to be saved from death, ie. salvation). Therefore, in order to be saved, one must have the Holy Spirit within them. Not only is having the Spirit what saves us the Scripture tells us that happens at the moment we believe, not the moment we are baptized with water.
Ephesians 1:13 And you also were included in Christ when you heard the word of truth, the gospel of your salvation. Having believed, you were marked in him with a seal, the promised Holy Spirit, 14who is a deposit guaranteeing our inheritance until the redemption of those who are God's possession--to the praise of his glory.
The passage in Romans makes it clear that having the Holy Spirit means we are saved and Ephesians tells us that happens the moment we believe. Having the Holy Spirit is how we know we are saved. Therefore, in order for water baptism to be required for salvation no one would be able to receive the Holy Spirit without having first been baptized with water. There could be no exceptions to this rule. Yet in Acts we see this very thing happen.
Acts 11:47 "Can anyone keep these people from being baptized with water? They have received the Holy Spirit just as we have." 48 So he ordered that they be baptized in the name of Jesus Christ. Then they asked Peter to stay with them for a few days.
These people had received the Holy Spirit before they were baptized with water. In fact it was because they already were saved that they were ordered to be baptized. They were not ordered to do so to receive salvation, they already had that because the Spirit already lived within them. It was because they had the Spirit that no one was able to stop them from being baptized with water. It is apparent that salvation can and should precede water baptism. To suggest otherwise is to ignore the teachings of Romans 8, Acts 11 and Ephesians 1 among others.
Too often the doctrine is confused because some people are too quick to impose the word water upon every Scripture that mentions the word baptism. We must remember that the word baptism does not always imply water baptism. It is the spiritual baptism that happens at salvation that makes us part of the body of Christ. Water has nothing to do with it.
1 Corinthians 12:13 For we were all baptized by one Spirit into one body
If the Spirit lives within you, you belong to Christ, end of story. This applies even if you have not been baptized by water, which was the case in Acts chapter 11. There are many things that are expected of us as Christians after we are saved. One is to be baptized with water, another is to observe the cup and bread, and yet another is to be witnesses for Christ. The list goes on. None of these things are required to be saved nor will they keep us saved. They are simply evidence that we already have been. We were saved and then created to do good works, it is not the other way around.
Ephesians 2:8 For it is by grace you have been saved, through faith--and this not from yourselves, it is the gift of God-- 9 not by works, so that no one can boast. 10 For we are God's workmanship, created in Christ Jesus to do good works, which God prepared in advance for us to do.
To learn more please see our article on Baptism.