Paul exhorts us through his letter to Titus reminding us that he saved us, washed us by regeneration, and renews us by the Holy Spirit.  This is a paraphrase, but it certainly sounds as if those who are baptized are those who necessarily have the Holy Spirit.  If this is a sign that affirms our possession of the Holy Spirit why would we apply this sign of baptism to infants when we cannot know if they have the Holy Spirit?