Anti-rejection drugs are daily medications taken by organ transplant patients to prevent organ rejection. Such drugs, also called immunosuppressants, help to suppress the immune system's response to a new organ. When a new organ is placed inside a patient's body, the patient's immune system recognizes the organ as foreign tissue and tries to reject it.
A similar thing happens when God puts a 'new heart' in us by '... the washing of regeneration and renewal of the Holy Spirit, whom he poured out on us richly through Jesus Christ our Savior' (Titus 3.5-6). For the rest of our earthly lives, our sinful nature will try to expel the 'new nature' that God has given us from our souls. We need to take 'anti-rejection medication', so to speak. That would be what Christians call the 'Means of Grace' - God's word and sacraments.
These are powerful spiritual agents that remain outside of us and useless to us unless taken religiously (if you pardon the pun). Those who think they can remain Christians, yet not receive the saving benefits of the Means of Grace are like transplant patients who refuse to take their anti-rejection meds. They need to ‘repent’ of not taking their meds, if you see what I mean. Otherwise there is a real danger that they will eventually become spiritually ill and reject the Spirit of God and the new life that was given to them to provide eternal life.
The baptism of John the Baptiser differed from the baptism of Jesus in that John's baptism brought the newly baptized to look forward to the Messiah who was to come and bring about total forgiveness by what He would later accomplish.