John Calvin Commentary Acts 6:4

"But we will continue stedfastly in prayer, and in the ministry of the word." — Acts 6:4 (ASV)

And we will give ourselves unto prayer. They show again that they have too much business otherwise, in which they may exercise themselves during their whole life. For the old proverb agrees with this very fittingly, which was formerly used in the solemn rites, do this. Therefore, they use the word [προσκαρτερησαι], which signifies to be, as it were, fastened and tied to anything.

Therefore, pastors must not think that they have so done their duty that they need to do no more when they have daily spent some time in teaching. There is another manner of study, another manner of zeal, another manner of continuance required, so that they may indeed boast that they are wholly given to that thing.

They add prayer to this, not that they alone should pray (for that is an exercise common to all the godly), but because they have specific reasons to pray above all others. There is no one who should not be careful for the common salvation of the Church. How much more, then, should the pastor, who has that function specifically assigned to him, labor carefully [anxiously] for it?

So Moses did indeed exhort others to prayer, but he went before them as their leader (Exodus 17:11). And it is not without cause that Paul so often makes mention of his prayers, (Romans 1:10). Again, we must always remember that we shall lose all our labor spent on plowing, sowing, and watering, unless the increase comes from heaven, (1 Corinthians 3:7).

Therefore, it will not be enough to take great pains in teaching, unless we ask for the blessing from the Lord, so that our labor may not be in vain and unfruitful. By this it appears that the exercise of prayer is not commended in vain to the ministers of the word.