I always assumed it was at St Michaels on the corner of Adshead Road and Queens Drive.
I remember back in the early 60's before that junction layout got modified, sometimes being taken down that route from West Derby many times by my Dad to get to to my Nans at Kemlyn and that church always had large posters up back then exhorting sinners to repent and be saved or God will have his mighty vengeance, and other variations on that theme.
I suppose without hard evidence, it's likely several churches around the general Anfield vicinity could probably lay claim, and quite possibly, it was a poster modified on more than one of them.