Let us settle it firmly in our minds that the Lord's Supper was not given to be a means either of justification or of conversion. It was never meant to give grace where there is no grace already, or to provide pardon when pardon is not already enjoyed. It cannot possibly provide what is lacking with the absence of repentance to God, and faith toward the Lord Jesus Christ. It is an ordinance for the penitent, not for the impenitent, for the believing, not for the unbelieving, for the converted, not for the unconverted. The unconverted man, who fancies that be can find a "shortcut" to heaven by taking the Lord's Supper, without treading the well-worn steps of repentance and faith, will find to his cost one day, that he is totally deceived. The Lord's Supper was meant to increase and help the grace that a man has, but not to impart the grace that he does not have. It was certainly never intended to make our peace with God, to justify, or to convert.
The simplest statement of the benefit which a truehearted communicant may expect to receive from the Lord's Supper, is the strengthening and refreshing of our souls--clearer views of Christ and His atonement, clearer views of all the offices which Christ, fills as our Mediator and Advocate, clearer views of the complete redemption Christ has obtained for us by His substituted death on the cross, clearer views of our full and perfect acceptance in Christ before God, fresh reasons for deep repentance for sin, fresh reasons for lively faith--these are among the leading returns which a believer may confidently expect to get from his attendance at the Lord's Table. He that eats the bread and drinks the wine in a right spirit, will find himself drawn into closer communion with Christ, and will feel to know Him more, and understand Him better. Taken in part from J.C. Ryle's book The Lord's Supper.
Other parts of his letter:
Why was the Lord’s Supper ordained?
Who ought to receive the Lord’s Supper?
What benefits are in receiving the Lord’s Table
What effects are in receiving the Lord’s Table
Warnings concerning the Lord’s Table