If a king came to Timothy and asked Timothy to pray for him, that then would be entirely biblical, because this would be exactly what the scripture tells Timothy to do.
If Timothy were to die, and go to heaven, Timothy could continue doing the same thing. That is because Timothy would then receive eternal life promised him in 1 Timothy 6. But now the king would be praying to St. Timothy, would he not?
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That's a very, very different thing, Biblically.
Two mortals praying for one another are
1. obeying specific Scriptural commands to do so. There is NO SUCH SCRIPTURAL COMMAND RE DEPARTED BELIEVERS.
2. Strengthening the koinonia in the mortal fellowship.
3. Strengthening their loving caring for one another.
4. Building up the local congregation by prayer and by the building up of Godly mortal relationships.
5. Most likely, Not infecting one another with adoration for post death supernatural powers or hotlines to The Father.
6. Not robbing Christ of dialogue, adoration and petitioning due Him which His Blood alone paid for.
And the king on earth asking Timothy in heaven to intercede to Christ is different how?