Speaking the Truth Plainly

Why did Paul feel he needed to remind them how they heard the Gospel? Does God use trials and pressures to place His people in the exact place they need to be? Do we allow God to do anything? Why is it vital that we speak of Christ with simplicity and plainness? Should you explain imputation and righteousness when speaking to those who are yet ignorant? How is repentance a change of heart toward Christ and His finished work? Are we all called to witness of Christ? Is any message of Christ preached in vain? What does contention mean here?