I think it came about through an initially biblical understanding.
1. The dear departed are alive and with the Lord.
2. We are surrounded by a great cloud of witnesses.
3. There is a conversation between Abraham and the Rich Man who hadn't helped Lazarus.
4. John is guided through the vision by one he starts to worshp who tells him not to do so, because he is numbered among John's brethren.
5. Revelation depicts souls under the altar who speak out to God.
6. Revelation 8 "Re 8:3 - Another angel, who had a golden censer, came and stood at the altar. He was given much incense to offer, with the prayers of all the saints, on the golden altar before the throne."
This understanding of believers alive with the Lord led to the notion that perhaps they could lobby the Almighty.
It never hurt to know someone who had the ear of the King.
This falls apart because these departed believers are not omniscient.
"God became man, that man might become God." Anastasius
This falls apart because these departed believers are not omniscient.
= = =
I think it also falls apart in that:
1. Heavenly residing believers have other God given priorities.
2. God is not interested--is likely very averse--to sharing HIS ROLE with anyone . . . CHRIST IS
THE
INTERCESSOR. Sharing His glory and role as that is not part of the script.