Ex opere operato is a Latin term literally meaning "from the work performed". This refers to the grace infallibly effected by the proper performance of the sacraments as ordained by God, and established by Christ as efficient causes of infused divine grace in and of themselves, independent of the worth or goodwill of the person administering the sacrament. An example is the sign of the washing of "regenerative baptism" (including pedobaptism) which "saves you now" (Ephesians 5:25-27; John 3:3-5; Titus 3:5; 1 Peter 3:21).
Ex opere operantis is a Latin term literally meaning "from the work of the doer". This refers to the grace or blessing of God determined solely by the faith and love of God exercised by the person, such as believing God as an act of righteousness, and as the fruit of righteousness by faith doing works of faith. An example is the symbolic ordinance of "believers' baptism" (credobaptism), "For by grace you have been saved through faith...not of works, lest any man should boast". (Romans 3:21-30; Acts 2:38; 8:37; Ephesians 2:4-10; James 2:14-26).