Mutual prayer among the living gives no warrant for invoking the departed
Even if the Roman Church asked nothing of the saints except that they pray to God for us, that could not be defended as lawful by the example of the prayers believers ask of one another, when under some need they ask their brothers to help them before God with their prayers.
Scripture tells us both to help our brothers with our prayers and to ask the same help from them. It does not follow that someone living in Europe could, without folly or even impiety, speak aloud or in his heart to a holy man living among the Chinese or in the Americas, ask him as if he could see and hear to intercede with God for him, and obtain what he feels he needs.
How then can we conclude from it that we may piously and reasonably ask the saints who have died in Christ, aloud or only in the heart's desire, to pray to God for us? They have been carried up into the heavenly paradise and are as far from us as the highest heaven is from the earth. Until we enjoy the same blessedness, we have no dealings with them and no means of sharing our thoughts.
So from the prayers that believers fighting side by side on earth ask of one another, it no more follows that we may invoke the saints already received into heaven than it once followed, on the Roman Church's own teaching, that the believers then living should have invoked the souls of the fathers held in Limbo.
Latin
XCV. Etiamsi vero Ecclesia Romana non aliud a Sanctis petere quam ut Deum pro nobis orent, istud tamen ut legitimum minime defendi posset exemplo precum illarum quas fideles a se invicem postulant; dum scilicet, aliqua necessitate pressi, fratres suos rogant ut ipsos apud Deum orationum suarum auxilio juvare velint. Nam ex eo quod in Scriptura monemur & fratres nostros precibus adjuvare, & mutuum ejusmodi auxilium ab ipsis petere, non sequitur aliquem in Europa degentem posse absque stultitia, aut etiam impietate, ore vel corde compellare virum Sanctum commorantem apud Sinas aut Americanos, eumque quasi videntem & audientem rogare, ut apud Deum pro ipso intercedat, & quae sentit sibi opus esse impetret. Quomodo ergo inde concludemus nos posse pie & cum ratione, a Sanctis in Christo defunctis postulare, seu voce, seu etiam solo cordis affectu, ut Deum pro nobis precentur; cum illi jam in caelestem paradisum evecti, tantum a nobis absint, quantum summum a terra caelum; nec ullum, donec simili beatitudine fruamur, supersit nobis cum ipsis commercium; & ratio mutuos conceptus communicandi? Itaque ex precibus quas a se mutuo postulant fideles in terra simul Militantes, non magis sequitur Sanctos in caelum jam receptos a nobis invocari posse, quam olim sequebatur, secundum doctrinam Ecclesiae Romanae, animas patrum in Limbo detentas a fidelibus tum temporis viventibus fuisse invocandas.