If merits of Christ are infinite, then we cannot do anything, need not do anything.

This forgets the syn Christo theme of St. Paul e.g, Rom 8:17:"We are heirs together with Christ, provided we suffer with Him so we may be glorified with Him". cf. also Romans 6:3-8; Col 3:1-3; Eph 2:5-9; Rom 8:9. We get in on the claim He established insofar as we are members of Him and like Him.