As we partake of the Lord’s Supper, we must remember that God is dealing with us here. What we do, what we eat, what we drink, alters us. It does not do this through magical means, but it is done through the power of the Holy Spirit, as He deals us covenantally.
One of the things He deals with is secret sin. Realize that partaking of this Supper is a public act—elders see you taking or trying to refrain, and the other members of the congregation should be looking around during their participation in the Supper, noting that they are communing together with you. This is not a private time of communion with God; this is a public time of communion with His saints, and therefore with Him.
God promises that He uses this means to deal with us as recuperating sinners. Most of us come knowing this, and submitting to it gladly. But some cherish secret sins, secret lusts, secret betrayals, secret thefts, and so on. For them, the Lord’s Supper is like an emetic. The more they try to keep the sin down and hidden, the more the authority of this Supper insists upon that sin coming to light. The Lord spoke through Moses centuries ago, saying, “Be sure your sin will find you out.”
For those who come to this Supper humbly, there is nothing to fear, and much to rejoice in. But for those who want to live for themselves in the depths of their own hearts, playing games with God’s holiness in public, it is a different story.