Louth on Wikipedia.
 

Chapel in the County Hospital on High Holme Road. Interior view. TF 325 879. Both © Mike Berrell (2012).

Christian Science Reading Room on Eastgate. Perhaps disused?. TF 329 874. © Mike Berrell (2012).

Eastgate Union Church (URC/Baptist) on Eastgate. Two interior views - 1, 2. TF 332 875. All © Mike Berrell (2012). Link.

Holy Trinity Church Centre on Eastgate. The church was destroyed in a fire in 1991, and the new centre was opened in 1997. Both events are recorded in a plaque. TF 336 877. © Mike Berrell (2012). Link.

Kingdom Hall of the Jehovah's Witnesses on Monks Dyke Road. TF 333 873. © Mike Berrell (2012).

Louth Christian Fellowship meet at St Michael's School on Monks Dyke Road (TF 334 872), and also at Cannon Street House on Cannon Street (TF 328 875). This was previously Congregational. Both © Mike Berrell (2012). Link. Since Mike took his photo, the church has been re-named as Generations Church (personal communication, 2019). The Cannon Street building is also listed on Genuki as a former Congregational church (and says it was founded in 1820). Howard Richter has advised that a map of 1889 has it labelled as a Sunday School for the adjacent Baptist Chapel (General).

Louth Evangelical Church on Monks Dyke Road. TF 337 874. © Mike Berrell (2012).

Louth Methodist Church Centre on Nichol Hill, was built as Wesleyan in 1835. TF 328 875. © David Regan (2011). The church was extensively altered and re-dedicated in 1977 (plaque). Another view, showing its main entrance on Eastgate, round the corner from where the first photo was taken. Interior view. All © Mike Berrell (2012). Link. Grade II listed - link.

Former Methodist Church on Virginia Drive. This building has been sold, and is to be demolished. TF 342 872. © Mike Berrell (2012).

Mortuary Chapel (one of two, both 1855) in Louth Cemetery on London Road. The second one is used by the Orthodox Community of St. Aetheheard. TF 330 868. Both © Mike Berrell (2012).

St. James on Westgate. The spire. Interior view. At just under 300 feet, the spire is the tallest of any medieval parish church in England. TF 326 874. All © Tudorbarlow (Flickr). Another view and interior view, both © Stuart Mackrell. Another view, © Mike Berrell (2012). Link. Grade I listed - link.

St. Mary (R.C.) on Upgate. TF 328 871. Three interior views - 1, 2, 3. All © Mike Berrell (2012). Link.

St. Michael & All Angels on Church Street. TF 333 871. © Dave Hitchborne. Another view, and two interior views - 1, 2, all © Mike Berrell (2012). Link.

Salvation Army (Louth Corps) on Church Street. TF 332 874. © Mike Berrell (2012).

Spiritualist Community Centre (Independent Spiritualist) on Mercer Row. TF 328 873. © Mike Berrell (2012). Link.

Former Wesleyan Methodist Chapel on Commercial Road, now in secular use. TF 336 878. © Mike Berrell (2012).
 

Howard Richter has identified some additional Louth churches, as follows -

As well as the former Congregational/Baptist chapel on Cannon Street (listed above), there was another Congregational chapel a little way further south on Cannon Street at TF 3283 8747. The Playhouse Cinema which now stands on the site can be seen on Streetview.

A Primitive Methodist Chapel stood on Northgate at TF 3297 8753, part of the public library now occupies the site. It can be seen on Streetview - the block in the middle distance with four ground floor windows.

Eastgate once had a large Free United Methodist Church at TF 3312 8747. A car park now occupies the site - seen here on Streetview.

Another small Free United Methodist Church still stands on Church Street at TF 3328 8694. Streetview here. Converted to residential use, it has recently been offered for rental. On the Streetview image a blue plaque can be seen, although I've so far been unable to discover what it commemorates.