Most people in the town identify as Christian, although the proportion has declined between the last two censuses.
At the 2011 Census, 70.3% of residents identified as Christian, while 21.8% reported no religion, and 6.6% did not state a religion; no other religious group comprised 1% or more of the population.
The 2001 Census recorded that 81.6% of Sleaford residents identified as Christian, nearly ten percentage points higher than the national figure (71.8%); 11.5% of the town's residents had no religion and 6% did not disclose a religion.

In the Compton Census (1676), New Sleaford had a Conformist population of 576 people, no "Papists" and 6 Non-conformists.
In the 19th century, it had a sizeable Non-conformist population and a large Anglican congregation; at the 1851 Census of Religious Worship, an estimated 2,000 people attended Non-conformist places of worship, while an estimated 600–700 people attended Anglican services in the parish.
The Wesleyans met in Westgate in the early 19th century; by 1848, the congregation had set up in Northgate, an area known for its taverns and poor tenements.
Most people in the town identify as Christian, although the proportion has declined between the last two censuses. At the 2011 Census, 70.3% of residents identified as Christian, while 21.8% reported no religion, and 6.6% did not state a religion; no other religious group comprised 1% or more of the population. The 2001 Census recorded that 81.6% of Sleaford residents identified as Christian, nearly ten percentage points higher than the national figure (71.8%); 11.5% of the town's residents had no religion and 6% did not disclose a religion.
In the Compton Census (1676), New Sleaford had a Conformist population of 576 people, no "Papists" and 6 Non-conformists. In the 19th century, it had a sizeable Non-conformist population and a large Anglican congregation; at the 1851 Census of Religious Worship, an estimated 2,000 people attended Non-conformist places of worship, while an estimated 600–700 people attended Anglican services in the parish. The Wesleyans met in Westgate in the early 19th century; by 1848, the congregation had set up in Northgate, an area known for its taverns and poor tenements.
Sleaford