But (alla). So usually, though it is open to question whether alla is adversative here and not rather, “Now then.”
Get thee down (katabēthi). Second aorist active imperative, at once.
Go (poreuou). Present middle imperative, go on.
Nothing doubting (mēden diakrinomenos). Another compound of dia, old and common verb for a divided mind (dia like duo, two). Note usual negative of the present middle participle, the subjective mēden. The notion of wavering (Jam 1:6) is common with this verb in the middle voice. In Act 11:12 the aorist active (mēden diakrinanta) is used perhaps with the idea of conduct towards others rather than his own internal doubt as here (Page).
For I (hoti egō). The Holy Spirit assumes responsibility for the messengers from Cornelius and thus connects their mission with the vision which was still troubling Peter. Peter had heard his name called by the man (Act 10:19).