Turretin on Prayers to the Dead

If I ask my living friends to pray for me, why shouldn’t I ask my friends who have died, or the other dead, to pray for me too?

invocation of saints has neither a precept, nor promise, nor example in Scripture for its foundation. Thus it is nothing else than a corrupt and damnable will-worship (ethelothre_skeia). Invocation of God is indeed everywhere urged, but mention is nowhere made of the invocation of creatures. (Francis Turretin, Institutes of Elenctic Theology, 11.7.2, v2 p42)

… invocation supposes … knowledge … in the one invoked: knowledge to know all our necessities and everywhere hear our prayers (mental as well as oral) and be truly a searcher of hearts (kardiognwte_s)…. Now these belong to God alone (the one searcher of hearts [kardionwste_s], who alone is the object of faith and omniscient….. As to the saints, on the contrary, we have no proof of their knowledge…. Indeed, Scripture (which often ascribes to God alone omniscience, heart-searching [kardiognwsian] and omnipotence, 1 K. 8:39; 2 Ch. 6:30; Rev. 2:23; Jer. 17:10) clearly proved that these cannot belong to creatures and frequently ascribes to the saints and ignorance of the affairs of earth (Is. 63:16; Ecc. 9:6; 2 K. 22:20). 11.7.3 p43

From the mutual prayers of the living to prayers of the departed for the living, the consequence does not hold good. Among the former, there is a mutual communion of offices and necessities; there is none between the dead and the living. The former are enjoined and praised in Scripture, but the latter by no means. Nor does the perfection of charity (with which the saints are blessed) conduce their praying for us particularly because the condition of that life does not allow of it. 14.4.xxii, v2 p390