John Calvin Commentary Acts 20:31

"Wherefore watch ye, remembering that by the space of three years I ceased not to admonish every one night and day with tears." — Acts 20:31 (ASV)

For which cause, watch. Paul again exhorts them to diligence by his own example, though he also joins with it a fear of danger, as if to say that they need great attention to be on guard; and that it is an unfitting thing that they should become weary, who had seen his remarkable patience for three years.

Also, he speaks of his tears, which added no small efficacy to his exhortations. When he says that he admonished everyone, this may refer both to the common people and to the elders. For since he was determined to say things applicable to the whole Church, he speaks as if the whole body were present.

Nevertheless, if anyone would rather restrict this admonition to the order of pastors, the meaning would be this: their zeal must be kindled not only by the speech he is now making, but it is also fitting that they remember those frequent exhortations which he continually impressed upon them for three years, and with many tears. Yet it seems more likely to me that he speaks of all in general.