Short answer:
The Church is an extension of the Incarnation united by the Eucharist.
Long answer:
catholic.com/documents/pillar-of-fire-pillar-of-truth
Longer answer:
vatican.va/archive/ccc_css/archive/catechism/p123a9p3.htm
Longest answer: follow the footnotes of the longer answer:
256 LG 8.
257 Cf. DS 2888.
258 Vatican Council I, De Filius 3

S 3013.
259 UR 2 § 5.
260 GS 78 § 3.
261 UR 2 § 2.
262 St. Clement Of Alexandria, Pæd. 1,6,42

G 8,300.
263 LG 13 § 2.
264 Eph 4:3.
265 Col 3:14.
266 Cf. UR 2; LG 14; CIC, can. 205.
267 LG 8 § 2.
268 UR 3 § 5.
269 UR 3 § 1.
270 Cf. CIC, can. 751.
271 Origen, Hom. in Ezech. 9,1

G 13,732.
272 UR 3 § 1.
273 LG 8 § 2.
274 UR 3 § 2; cf. LG 15.
275 Cf. UR 3.
276 Cf. LG 8.
277 UR 4 § 3.
278 Jn 17:21; cf. Heb 7:25.
279 Cf. UR 1.
280 Cf. UR 6.
281 UR 7 § 3.
282 UR 8 § 1.
283 Cf. UR 9.
284 Cf. UR 10.
285 Cf. UR 4; 9; 11.
286 Cf. UR 12.
287 UR 5.
288 UR 24 § 2.
289 LG 39; Cf. Eph 5:25-26.
290 LG 12.
291 Acts 9:13; 1 Cor 6:1; 16:1.
292 SC 10.
293 UR 3 § 5.
294 LG 48.
295 LG 48 § 3.
296 LG 11 § 3.
297 LG 42.
298 St. Thérèse Of Lisieux, Autobiography of a Saint, tr. Ronald Knox (London: Harvill, 1958) 235.
299 LG 8 § 3; Cf. UR 3; 6; Heb 2:17; 726; 2 Cor 5:21.
300 Cf. 1 Jn 1:8-10.
301 Cf. Mt 13:24-30.
302 Paul VI, CPG § 19.
303 Cf. LG 40; 48-51.
304 John Paul II, CL 16,3.
305 CL 17, 3.
306 LG 65; cf. Eph 5:26-27.
307 St. Ignatius of Antioch, Ad Smyrn. 8,2:Apostolic Fathers,II/2,311.
308 UR 3; AG 6; Eph 1:22-23.
309 Cf. AG 4.
310 Cf. Mt 28:19.
311 LG 13 §§ 1-2; cf. Jn 11:52.
312 LG 26.
313 Cf. CD 11; CIC, cann. 368-369; CCEO, cann. 171,1; 178; 311,1; 312.
314 LG 23.
315 St. Ignatius Of Antioch, Ad Rom. 1,1:Apostolic Fathers,II/2,192; cf. LG 13.
316 St. Irenaeus, Adv. haeres. 3,3,2

G 7/1,849; Cf. Vatican Council I

S 3057.
317 St. Maximus the Confessor, Opuscula theo.

G 91:137-140.
318 Paul VI, EN 62.
319 LG 23.
320 LG 13.
321 LG 14.
322 LG 15.
323 UR 3.
324 Paul VI, Discourse, December 14, 1975; cf. UR 13-18.
325 LG 16.
326 Cf. NA 4.
327 Roman Missal, Good Friday 13:General Intercessions,VI.
328 Rom 9:4-5.
329 Rom 11:29.
330 LG 16; cf. NA 3.
331 NA 1.
332 LG 16; cf. NA 2; EN 53.
333 LG 16; cf. Rom 1:21, 25.
334 St. Augustine, Serm. 96,7,9

L 38,588; St. Ambrose, De virg. 18 118

L 16,297B; cf. already 1 Pet 3:20-21.
335 Cf. Cyprian, Ep. 73.21

L 3,1169; De unit.

L 4,509-536.
336 LG 14; cf. Mk 16:16; Jn 3:5.
337 LG 16; cf. DS 3866-3872.
338 AG 7; cf. Heb 11:6; 1 Cor 9:16.
339 AG 1; cf. Mt 16:15.
340 Mt 28:19-20.
341 AG 2.
342 Cf. John Paul II, RMiss 23.
343 2 Cor 5:14; cf. AA 6; RMiss 11.
344 1 Tim 2:4.
345 John Paul II, RMiss 21.
346 AG 5.
347 Tertullian, Apol. 50,13

L 1,603.
348 GS 43 § 6.
349 LG 8 § 3; 15; AG 1 § 3; cf. RMiss 12-20.
350 LG 8 § 3.
351 GS 40 § 2.
352 Cf. RMiss 42 47.
353 AG 15 § 1.
354 Cf. RMiss 48-49.
355 Cf. RMiss 52-54.
356 AG 6 § 2.
357 Cf. RMiss 50.
358 UR 4 § 8.
359 Cf. RMiss 55.
360 AG 9.
361 AG 9.
362 Eph 2:20; Rev 21:14.
363 Cf. Mt 28:16-20; Acts 1:8; 1 Cor 9:1; 15:7-8; Gal 1:1; etc.
364 Cf. Acts 2:42.
365 Cf. 2 Tim 1:13-14.
366 AG 5.
367 Roman Missal, Preface of the Apostles I.
368 Mk 3:13-14.
369 Jn 20:21; cf. 13:20; 17:18.
370 Mt 10:40; cf. Lk 10:16.
371 Jn 5:19, 30; cf. Jn 15:5.
372 2 Cor 3:6; 6:4; 5:20; 1 Cor 4:1.
373 LG 20; cf. Mt 28:20.
374 LG 20; cf. Acts 20:28; St. Clement of Rome, Ad Cor. 42,44

G 1,291-300.
375 LG 20 § 2.
376 LG 20 § 2.
377 AA 2.
378 AA 4; cf. Jn 15:5.
379 AA 3.
380 Rev 19:6.
381 Eph 1:4.
382 Rev 21:9.
383 Rev 21:10-11.
384 Rev 21:14.
This tells you what those abbreviations mean:
vatican.va/archive/ccc_css/archive/catechism/abbrev.htm
A church is a building where the faithful meet. A church is also a community of believers. The ISIS can blow up churches and behead children, but they can’t destroy their faith.
THAT is a CHURCH!