This post was taken from another thread where it was off topic.
Since I cannot come up the best way of explaining this on my own I will let another say it for me.
Christ merited for us all the grace we need to attain our supernatural end and to develop in us the supernatural life: “Who has blessed us with spiritual blessings in heavenly places in Christ.” (Ephesians 1:3) He merited for us the grace of conversion, the grace of steadfastness in good, the helps to resist temptation, the aids to profit by trial, the grace of comfort in the midst of tribulations, the grace of renewal of spirit and of final perserverance. He merited all things for us. We have the solemn word that anything we ask the Father in His name, that is, through His own merits, will be granted to us. (John 16:23) Then in order to inspire us with greater confidence, He instituted the sacraments, visible signs, which confer His grace in all the important events of life and which give us a right to actual graces in time of need.
He has gone further still. In His desire to associate us with Himself in the work of of our own sanctification, He has given us the power of satisfying and meriting, thus making us the secondary causes, the agents of our own sanctification. He has, as a matter of fact, made this co-operation a law and an essential condition of our spiritual life. If He carried His cross, it is that we may follow Him bearing ours: “If any man will come after me, let him deny himself and take up his cross and follow me.” (Matthew 16:24) It was thus understood by the apostles. If we would share in His glory, says St. Paul, we must share in His sufferings: “Yet so, if we suffer with Him, that we may be also glorified with him.” (Romans 8:17) St. Peter adds that if Christ suffered for us it is that we may follow in His footsteps. (1 Peter 2:21) Moreover, self sacrificing souls are urged, after the manner of the apostle of the Gentiles, to undergo suffering joyfully in union with Christ for the sake of the Church, His mystical body: “Who now rejoice in my sufferings for you and fill up those things that are wanting of the sufferings of Christ, in my flesh, for his body, which is the church.” (Collosians 1:24) In this way these souls share in the redeeming power of Christ’s passion and become secondary agents of the salvation of their brethren. How true, how sublime, how consoling is this doctrine! Compare it with the incredible affirmation of certain Protestants who assert, that since Christ suffered to the full for us, there remains for us only to enjoy the fruits of His plentiful redemption without drinking of His chalice. They thus pretend to pay homage to the fullness of Christ’s merits. Does not our Christ-given power to merit show forth better the fullness of the redemption by Christ? Does it not do more honor to Christ to manifest the power of His satisfaction by enabling us to join in His work of atonement and co-operate with Him even though in a secondary manner?" - “The Spiritual Life” A. Tanquerey #'s 134 & 135