Scripture reading: Matthew 5:27-30

For today’s rounds, we return to the familiar field of obstetrics, reflecting on rhesus sensitisation.

A rhesus-negative mother may feel completely well after exposure to her fetus’s rhesus-positive blood. There is no fever. No pain. No outward sign that anything lasting has occurred. Yet quietly, within her immune system, something permanent has taken place: memory has formed.

The next exposure will not be silent. It will be swift. Forceful. Harmful. The consequences may unfold as recurrent miscarriages, fetal anaemia, hydrops fetalis, intrauterine fetal demise, or hemolytic disease of the fetus and newborn. To prevent this, pregnant women are screened for their Rhesus status. If a mother is Rh-negative and the baby may be Rh-positive, she receives anti-RhD immunoglobulin to stop sensitisation before it begins.

In today’s passage, Jesus reshapes our understanding of sin. It is not merely the outward act that matters; it begins in the realm of desire.

Small exposures can carry far-reaching effects.

In Song of Solomon 2:15, Scripture urges: “Catch the little foxes, the little foxes that spoil the vines…” Not wolves. Not storms. Not raging fires. Little foxes.

Vineyards are rarely destroyed overnight. Damage usually comes slowly, through small breaches, repeated nibbles, subtle erosion over time. So, it is with the soul. The first compromise seldom feels catastrophic. The first indulgence rarely feels fatal. The first step away hardly feels distant. But something forms inside: memory.

From there, progression is familiar. James 1:14–15 describes it plainly: desire is conceived, conception gives birth to sin, and sin matures into death. It begins quietly, internally, almost invisibly. No one sets out intending to ruin their integrity, marriage, or ministry. The path often begins with exposure that is entertained rather than resisted.

The enemy seldom presents the outcome. He offers only the first step. Just look. Just try. Just once. Just this small compromise.

Yet repeated exposure reshapes the heart. In immunology, sensitisation makes the immune system more reactive. Spiritually, repeated compromise often produces the opposite effect — dullness. Hebrews 3:13 cautions us not to be hardened through sin’s deceitfulness. Sin deceives because it hides its destination. It whispers that nothing has changed. But over time, what once convicted now entertains. What once disturbed now feels normal. What once shocked barely registers.

As 1 Timothy 4:2 describes, the conscience can become seared. Wrong remains wrong, but sensitivity fades.

Prevention is better than cure. Once Rh sensitization occurs, it cannot be reversed. Management becomes complicated. Risks increase. Spiritually, the stakes are even higher. Small patterns form strongholds. Private indulgences shape public outcomes. Tiny permissions grow into entrenched habits. No one drifts into holiness. Drift moves in the other direction.

But here the analogy reaches its limit, and grace begins. In medicine, sensitisation is permanent. In Christ, renewal is possible. 1 John 1:9 reminds us that if we confess our sins, He forgives and cleanses.