What is meant by eating this flesh and drinking this blood, which is so necessary and beneficial; it is certain that is means neither more nor less than believing in Christ. As we partake of meat and drink by eating and drinking, so we partake of Christ and his benefits by faith: and believing in Christ includes these four things, which eating and drinking do:--
First, It implies an appetite to Christ. This spiritual eating and drinking begins with hungering and thirsting (Matt. v. 6), earnest and importunate desires after Christ, not willing to take up with any thing short of an interest in him: "Give me Christ or else I die."
Secondly, An application of Christ to ourselves. Meat looked upon will not nourish us, but meat fed upon, and so made our own, and as it were one with us. We must so accept of Christ as to appropriate him to ourselves: my Lord, and my God, John xx. 28.
Thirdly, A delight in Christ and his salvation. The doctrine of Christ crucified must be meat and drink to us, most pleasant and delightful. We must feast upon the dainties of the New Testament in the blood of Christ, taking as great a complacency in the methods which Infinite Wisdom has taken to redeem and save us as ever we did in the most needful supplies or grateful delights of nature.
Fourthly, A derivation of nourishment from him and a dependence upon him for the support and comfort of our spiritual life, and the strength, growth, and vigour of the new man. To feed upon Christ is to do all in his name, in union with him, and by virtue drawn from him; it is to live upon him as we do upon our meat.
-Matthew Henry Commentary