Relationship between input voltage and output voltage for an active filter
Since you are talking about an active filter, it is assumed that the input signal consists of many frequencies. Also the gain of the active filter will vary with frequency; otherwise it is not a filter.  The relationship you have is only valid for a single frequency (Note that Av will vary with frequency for a filter). At that frequency the filter will have a gain and a phase shift.  However the overall response of the filter will have a gain and phase that varies with frequency. It is usually described with two graphs: one for gain versus frequency and one for phase shift versus frequency.