Colossians 4:12 Epaphras, who is one of you, a bondservant of Christ,
greets you, always laboring fervently for you in prayers,
that you may stand perfect and complete in all the will of God.
Since the Lord taught to pray "Thy Will Be Done", there can be no error
in praying for the same on behalf of others, for the scriptures state that
His Will is that all would be saved and come to the knowledge of the Truth.
What is that "effectual prayer". One that acknowledges and relies upon
the promises of the Father, as fulfilled by the Holy Spirit.
John 16:
8 And when He has come, He will convict the world of sin, and of righteousness, and of judgment:
The effectual, fervent prayer of a righteous person convicts the hearts of those on whose
behalf the intercession is made, by the power of the Holy Spirit. This is one part of our relationship
with the Lord.
13 However, when He, the Spirit of truth, has come, He will guide you into all truth; for He will not speak on His own, but whatever He hears He will speak; and He will tell you things to come.
14 He will glorify Me, for He will take of what is Mine and declare it to you.
15 All things that the Father has are Mine. Therefore I said that He will take of Mine and declare it to you.