The Collaborative International Dictionary of English v.0.48:
Effectual \Ef*fec"tu*al\ (?; 135), a. [See Effect, n.]
Producing, or having adequate power or force to produce, an
intended effect; adequate; efficient; operative; decisive.
Effectual steps for the suppression of the rebellion.
Effectual calling (Theol.), a doctrine concerning the work
of the Holy Spirit in producing conviction of sin and
acceptance of salvation by Christ, -- one of the five
points of Calvinism. See Calvinism.
Syn: Effectual, Efficacious, Effective.
Usage: An efficacious remedy is had recourse to, and proves
effective if it does decided good, effectual if it
does all the good desired. --C. J. Smith.