All believers have to be faithful. Jesus words in Matt 24:44-51 make this very clear:
“Therefore you also must be ready, for the Son of Man is coming at an hour you do not expect. (45) “Who then is the faithful and wise servant, whom his master has set over his household, to give them their food at the proper time? (46) Blessed is that servant whom his master will find so doing when he comes. (47) Truly, I say to you, he will set him over all his possessions. (48) But if that wicked servant says to himself, ‘My master is delayed,’ (49) and begins to beat his fellow servants and eats and drinks with drunkards, (50) the master of that servant will come on a day when he does not expect him and at an hour he does not know (51) and will cut him in pieces and put him with the hypocrites. In that place there will be weeping and gnashing of teeth.”
Preachers have to be especially faithful as they are representing Christ to many people and if they are unfaithful, the name of Christ may be brought into disrepute.
Consider James 3:1:
“Not many of you should become teachers, my brothers, for you know that we who teach will be judged with greater strictness. “
Paul’s instructions to Timothy about the character of overseers also shows the importance of preachers being faithful. 1 Tim 3:2-7:
“Therefore an overseer must be above reproach, the husband of one wife, sober-minded, self-controlled, respectable, hospitable, able to teach, (3) not a drunkard, not violent but gentle, not quarrelsome, not a lover of money. (4) He must manage his own household well, with all dignity keeping his children submissive, (5) for if someone does not know how to manage his own household, how will he care for God’s church? (6) He must not be a recent convert, or he may become puffed up with conceit and fall into the condemnation of the devil. (7) Moreover, he must be well thought of by outsiders, so that he may not fall into disgrace, into a snare of the devil.