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She meant Acts 2:38. As Winer says, p 494ff, "Grammar," Prepositions with the Accusative, III .... (c) in the context of metaphysical matters, eis means "an aim" of some purpose. Further, "Objective and subjective destination, result and purpose, are sometimes not to be separated, e. g., in H. iv. 16, L. ii. 34, Rom. xiv. 1, Jude 21."
Eis is a correlative preposition. I think Wallace is on the wrong track in engaging with "cause" (the ultimate cause is often "God" in metaphysics). Rather as Winer says, the intention is to show result and purpose are divinely instated. They are not to be, or cannot be, or should not be, separated by man. Thus the onus is on the church to baptize, the onus is on the believer to be baptized. Christianity was never conceived as a solo endeavor.