"Therefore" requires a look back to chapter 3 for context. Beginning in verse 7 and following we find a Contrast of the two Covenants (Law & Grace). Paul shows that the glory of the Law has passed, as did the glory of Moses' face, indicating that the glory of the New is greater; though it be unseen due to the blindness imposed by satan on those who do not believe. This blindness is due to rejecting Christ, Who is clearly seen in the Old Testament; but, that blindness is taken away when one believes.
"Therefore, since we have this ministry", i.e., of reconciliation via the Gospel...we have renounced the hidden things of shame..." Paul says that he and his company have refused to use manipulative words to gain converts (which would not be truly converted)...but, rather, they continue preaching the Gospel without deceit, trusting God to win souls to Himself via the work of the Holy Spirit.
I, personally, have always held to this philosophy, seeking to trust that God would do His work in conviction and conversion of souls rather than me manipulating "decisions" and responses to the invitation. Discouraging at times, yes, but trusting the work of the Spirit rather than resorting to manipulation, I hope to see His converts rather than mine.
The Bible warns of false teachers who use "feigned words... making merchandise of you..." (2 Peter 2:3). The Greek word translated "feigned" is "plastos", from which we get the word "plastic". It means "molded, artificial". False teachers profane the purity of the pulpit by using words that are specifically molded to their own purpose, which is to make merchandise (to buy and sell).
Pulpit "profiteers" abound today. Beware of them.
2 Peter 2:17-19
17 These are wells without water, clouds that are carried with a tempest; to whom the mist of darkness is reserved for ever.
18 For when they speak great swelling words of vanity, they allure through the lusts of the flesh, through much wantonness, those that were clean escaped from them who live in error.
19 While they promise them liberty, they themselves are the servants of corruption: for of whom a man is overcome, of the same is he brought in bondage.
Sing "How Firm A Foundation" to remind you of the stability of God's unadulterated Word.