Preceded by festival appearances and a headlining tour in the United States, "Writing on the Walls" was released as the first single from Define the Great Line on June 27, 2006.

Underoath spent the first half of the year touring the North America with Taking Back Sunday, Norma Jean, and Maylene and the Sons of Disaster.

[7] Drummer Aaron Gillespie said they had better distribution, while vocalist Spencer Chamberlain said that both Solid State and Tooth & Nail shared the same mailing list.

[11][13] While most commonly cited as metalcore[14][15][16][17][18] and emo,[19][20][21] the sound of Define the Great Line has also been tagged as Christian metal,[19] post-metal and post-hardcore.

[22] Indie Vision Music's Brandon J. noted influences Beloved, the Chariot and Maylene and the Sons of Disaster, while Chamberlain highlighted At the Drive-In and Glassjaw.

[25] Tony Cummings of Cross Rhythms described the album as "an impressively versatile project where metal riffs and emo breakdowns, screamo noise and memorable hooks collide in a veritable sonic feast,"[26] and AllMusic described the band as finding the centre point between "throat-shredding grindcore and My Chemical Romance/From Autumn to Ashes-style emo-punk".

[24] Define the Great Line was noted as a turning point for Underoath, with AllMusic commenting that "while the tendency to dissolve into the abyss of angtsy emo-pop was still there," there was a "darkness lurking in the nooks and crannies between the crackling snare hits and heavy 'drop-d' riffing" that would be fully realized on their sixth studio album Lost in the Sound of Separation (2008).

[22] For its lyrics, the band maintained its focus on Christianity and personal struggles, and according to Josh Taylor of Jesus Freak Hideout the big difference was "the frequency at which God is addressed by name," though all of these mentions were in "exclamations of desperation".

[19] Drowned in Sound writer Raziq Rauf said there was "no reason to treat the explicitly religious content" of the release differently from other emo albums, as it "covers the same bases of heartache, frustration and the inevitable exhaustion that results".

[21] The metalcore track "You're Ever So Inviting" is followed by "Sálmarnir", a song that consists of the Bible passage Psalm 50 being recited in Russian by a friend of the members.

[20][31][17] The title comes from the Icelandic word for "Psalms"; for its second half, Dudley is head repeating it in English in the manner of a Televangelist, backed by the crowd noise.

[17][33] "Returning Empty Handed" opens with a quiet instrumental passage, which gives way to a heavier section, before ending acoustically; over the course of its duration, it changes from metalcore to post-metal.

[34] On April 17, 2006, the release was titled Define the Great Line, and "Sálmarnir" was posted on the band's PureVolume account, followed by "Writing on the Walls" at the end of May, and "In Regards to Myself" in June.

[48][49] In July 2007, the band released their first video album titled 777, which included footage from the 2006 Warped Tour, Taste of Chaos and The Bamboozle Festival.

[51] Define the Great Line was included in the Play Your Old Stuff: An Underoath Anthology (2011) three-CD set, alongside their third studio album The Changing of Times (2002) and They're Only Chasing Safety.

[52] Define the Great Line was packaged with They're Only Chasing Safety as a two-LP set to promote the band's Rebirth Tour in 2016, during which they performed both albums in full.

[66] In May 2007, the band headlined the short Dirty South Tour in the US, with support from Norma Jean, Maylene and the Sons of Disaster, and the Glass Ocean, before playing a few shows in Australia with Emery in June 2007.

[68] Between late September and early November, the band went on a headlining US tour with support from Every Time I Die, Maylene and the Sons of Disaster, and Poison the Well.

[26][28] The Kern County Kid of HM called the album "nearly perfect," stating that it was "like one harsh line in the sand that dares its audience to embrace the band on its own terms".

[33] Monger described Define the Great Line as a "mammoth production that exemplifies how far Christian metal has come since the days of Stryper and Bloodgood", and called it "blistering and occasionally majestic".

[28] Farias said it said the album "sounds huge, far removed from the tamer and thinner feel of its predecessor" as "every component of the Underoath machine is amplified a hundredfold".

[22] Sputnikmusic staff member John Hanson said Chamberlain used to be an "atrocious screamer", and with the new album, "he has improved somewhat", while Gillespie was a "far more enjoyable vocalist to listen to, even though he could possibly be a tad bit generic".

[26] Rauf lambasted Chamberlain's "holier-than-thou preaches" that were in "fewer spurts and shrieks that we're used to," as he used a "greater range of impressive roars and screams" to convey them.

[20] Fish was thankful that Chamberlain was "no longer an indiscriminate, undecipherable yeller," but instead a "versatile vocalist" that can bounce "impeccably between feral shrieks, bestial growls, and melodic tones".

[21] Punknews.org staff member Brian Shultz noted that there was less singing than with the previous album, "thus providing a nice dynamic as opposed to a chorus".

[74] Define the Great Line was certified gold by the Recording Industry Association of America (RIAA) in November 2006 for selling 500,000 copies.

[77] Kerang listed Define the Great Line as one of the “21 Greatest Metalcore Albums of All-Time,” specifically highlighting Chamberlin’s “lacerating, confessional vocals” and Gillespie’s “melodic style” drumming.

[78] In 2023, Revolver ranked Define the Great Line at number 1 on its list of Underoath albums, praising the songwriting, musicianship, and the back and forth between Chamberlain and Gillespie.