If the Greek word "eis" ("for" in English) is always forward looking, then what about Matthew 3:11? Here it says that John baptized for repentance, but the repentance would have had to have come before the baptism. After all, Mark 1:4 and Luke 3:3 states John's baptism was a baptism of repentance. Therefore, "eis" can be "because of" in Acts 2:38.
The question shown above is actually a brief summary of a very convoluted argument sent to me. Rather than quoting reputable sources which state that eis is a preposition that looks forward to a goal and play the game of "my scholar is bigger than your scholar," I will try to address the question in a plain fashion.
In Matthew 26:28 Jesus stated, "For this is My blood of the new covenant, which is shed for many for the remission of sins." This particular verse is of interest because the phrase "for the remission of sins" is precisely the same Greek phrase used in Acts 2:38. Did Jesus shed his blood because men's sins had already been forgiven or was it shed with the view that men's sins can be forgiven? I hope that it is clear that Jesus' blood brought the forgiveness of sins; that forgiveness did not exist prior to Jesus' death. "For all have sinned and fall short of the glory of God, being justified freely by His grace through the redemption that is in Christ Jesus, whom God set forth as a propitiation by His blood, through faith, to demonstrate His righteousness, because in His forbearance God had passed over the sins that were previously committed" (Romans 3:23-25). Or in Ephesians 1:7, "In Him we have redemption through His blood, the forgiveness of sins, according to the riches of His grace." Therefore, Matthew 26:28 proves the possibility that "for (eis)" can be used in the forward looking sense.
The question then is did Peter use it in a forward looking sense or a backward sense as the questioner argues? Let us just for a moment assume that the questioner is correct (though the weight of scholarship is against him) and that eis might be translated "because of" in some cases and in Acts 2:38 in paticular.
In Acts 2 Peter had just finished laying out strong evidence that the Jews had murdered the Messiah. In response to this message we read, "Now when they heard this, they were cut to the heart, and said to Peter and the rest of the apostles, "Men and brethren, what shall we do?" " (Acts 2:37). Peter's message had got through to these people. What could they possibily do about this great sin that was theirs to bear? The questioner argues that Peter's response was "Repent, and each of you be baptized in the name of Jesus Christ because of the forgiveness of your sins." The questioner has Peter saying that they needed to repent and be baptized because they had already been forgiven of their sins. Now isn't that strange. The people asked what they needed to do to be saved and we are told that Peter didn't answer their question. Instead, we are told, that Peter told them they were already saved. Yet as we continue to read we find this, "And with many other words he testified and exhorted them, saying, "Be saved from this perverse generation"" (Acts 2:40). Wait a minute! Didn't the questioner have Peter telling the people they were already saved? Why does Peter continue to exhort the crowd to be saved afterwards? Then notice how the crowd responded to Peter's exhortation, "Then those who gladly received his word were baptized; and that day about three thousand souls were added to them" (Acts 2:41).
All Greek scholars admit that the Greek word eis is a preposition that looks forward to an object. A few, notably those persuaded that baptism is not essential for salvation, argue that in a few cases eis operates like our English word "for" which can look both forward and backward, depending on the context. However, it is obvious from the context of Acts 2:38 that Peter was not stating that his audience had already received salvation, else he would have had no need to continue to exhort them to be saved from their sins. Hence, Peter's use of eis in Acts 2:38 is the typical forward looking usage. A person who repents and is baptized can look forward to forgiveness of their sins. When the people accepted Peter's teaching, they were baptized and were added to the rolls of God's children. The implication is that they were forgiven of their sins; matching Peter's later statement, "baptism now saves you--not the removal of dirt from the flesh, but an appeal to God for a good conscience--through the resurrection of Jesus Christ" (I Peter 3:21).
Before ending this reply, let us now address whether eis in Matthew 3:11 must be referring to a past event. From a commentary written by J. W. McGarvey, Mr. McGarvey addresses this concern:
"The phrase under consideration has another meaning, though somewhat obscure as regards it connection with the facts, is very naturally expressed by the words themselves. The preposition is often expressive of purpose, and the phrase may be properly rendered 'in order to repentance.' The baptism was not in order to the repentance of the party baptized. ... But a baptism which required repentance as a prerequisite would have a tendency to cause those yet unbaptized to repent, in order that they might receive the baptism and enjoy its blessings. Prizes in schools are given in order to good behavior and good recitations, although the good recitations and good behavior must precede the reception of the prizes. Promotions in the army are in order to the encouragement of obedience and valor, although these qualitites of the good soldier must appear before the promoition can take place. In the same way was John's baptism in order to repentance. The inestimable blessing of remission of sins being attached to baptism (see Mark 1:4; Luke 3:3), the desire to obtain this blessing, would prompt those yet unbaptized to repent, so that they might be baptized. The words declare simply that the general purpose of John's baptism was to bring the people to repentance."
Hence, there is a way to read Matthew 3:11 that retains the common forward looking meaning of eis.