John 3:30 He must increase, but I must decrease.
John was the messenger sent to prepare the way for Jesus Christ. When Christ began his ministry and was baptised by John, the major part of John’s work was done.
John ‘decreased’ by being put into prison and eventually executed by Herod.
Christ ‘increased’ by fulfilling the purpose that God had assigned to him, being raised from the dead to the divine nature and sitting at the right hand of God. He will further ‘increase’ by returning to rule this earth in righteousness.
That is the literal situation. The spiritual principle illutstrated by John's statement in terms of ‘knowing God’ is that it is always the case that our personal "I" must decrease so that the "mind of Christ" - see Romans 8 - may increase in us.
I hope you have found this helpful.
May God bless you,