"Receive the grace of God"- immediately, so that the preaching of the Gospel to them is not "in vain", i.e. to no effect, for not being believed and received as God's message. The Gospel is not "for later" or "for any day" (as many say), but for today.
Not by letter, but by their character and by their sufferings for the love of the Gospel, Paul and his companions proved to be true servants of God:
He wanted their full love and trust, as children to their father.
1 We then, as workers together with Him also plead with you not to receive the grace of God in vain.
2 For He says: "IN AN ACCEPTABLE TIME I HAVE HEARD YOU, AND IN THE DAY OF SALVATION I HAVE HELPED YOU." Behold, now is the accepted time; behold, now is the day of salvation.
3 We give no offense in anything, that our ministry may not be blamed.
4 But in all things we commend ourselves as ministers of God: in much patience, in tribulations, in needs, in distresses,
5 in stripes, in imprisonments, in tumults, in labours, in sleeplessness, in fasting;
6 by purity, by knowledge, by longsuffering, by kindness, by the Holy Spirit, by sincere love,
7 by the word of truth, by the power of God, by the armour of righteousness on the right hand and on the left,
8 by honour and dishonour, by evil report and good report; as deceivers, and yet true;
9 as unknown, and yet well known; as dying, and behold we live; as chastened, and yet not killed;
10 as sorrowful, yet always rejoicing; as poor, yet making many rich; as having nothing, and yet possessing all things.
11 O Corinthians! We have spoken openly to you, our heart is wide open.
12 You are not restricted by us, but you are restricted by your own affections.
13 Now in return for the same (I speak as to children), you also be open.
2 Corinthians chapter 6, verses 1 to 13