"καὶ βαπτισθήτω ἕκαστος ὑμῶν ἐπὶ τῷ ὀνόματι Ἰησοῦ Χριστοῦ εἰς ἄφεσιν τῶν ἁμαρτιῶν ὑμῶν". Acts 2:38b
Eis is a preposition! Both "for" and "because of" are conjunctions. Plus, eis takes the accusative. It usually is "into, in, among" according to my mini dictionary, which is based on BDAG. However, Koine Greek does have some strange twists to it, as the language was in flux.
It can NOT be "because of" because εἰς takes the accusative, and ἄφεσιν is in the accusative. Anytime in English, you have "of" it automatically is known that it is genitive, which εἰς is not! It also can NOT be "because of," because εἰς is a preposition, and "because" is a conjunction.
So, I went to Bauer BDAG and the fifth meaning is:
5. Marker of a specific point of reference; for, to, with, with respect to, with reference to.
Because there is such a high frequency of εἰς, BDAG does not list this occurrence of εἰς in Acts 2:38. But, I would have to say Luke had a reason for writing it this way, not to be a conjunction, but as a point of reference.
One could even say: in the name of Jesus Christ, with reference to forgiveness of your sins.