Matthew 16:18
And <1161> (CONJ) I <2504> (P-1NS-C) say <3004> (V-PAI-1S) also <2504> (P-1NS-C) unto thee <4671> (P-2DS), That <3754> (CONJ) thou <4771> (P-2NS) art <1488> (V-PXI-2S) Peter <4074> (N-NSM), and <2532> (CONJ) upon <1909> (PREP) this <3778> (D-DSF) rock <3588> <4073> (T-DSF) (N-DSF) I will build <3618> (V-FAI-1S) my <3450> (P-1GS) church <3588> <1577> (T-ASF) (N-ASF); and <2532> (CONJ) the gates <4439> (N-NPF) of hell <86> (N-GSM) shall <2729> (V-FAI-3P) not <3756> (PRT-N) prevail against <2729> (V-FAI-3P) it <846> (P-GSF}.

Here, Jesus seems to basically say that he has not built his church yet. this goes against what my former church taught. Also, this is the first mention of the word "church".

Matthew 18:17
And <1161> (CONJ) if <1437> (COND) he shall neglect to hear <3878> (V-AAS-3S) them <846> (P-GPM), tell <2036> (V-2AAM-2S) it unto the church <3588> <1577> (T-DSF) (N-DSF): but <1161> (CONJ) if <1437> (COND) <2532> (CONJ)he neglect to hear <3878> (V-AAS-3S) the church <3588> <1577> (T-GSF) (N-GSF), let him be <2077> (V-PXM-3S) unto thee <4671> (P-2DS) as <5618> (ADV) an heathen man <1482> (A-NSM) and <2532> (CONJ) a publican <3588> <5057> (T-NSM) (N-NSM).

2nd mention of the word church.

Acts 2:47
Praising <134> (V-PAP-NPM) God <3588> <2316> (T-ASM) (N-ASM), and <2532> (CONJ) having <2192> (V-PAP-NPM) favour <5485> (N-ASF) with <4314> (PREP) all <3650> (A-ASM) the people <3588> <2992> (T-ASM) (N-ASM). And <1161> (CONJ) the Lord <2962> (N-NSM) added <4369> (V-IAI-3S) to the church <3588> <1577> (T-DSF) (N-DSF) daily <2250> (N-ASF) such <3588> (T-APM) as <2596> (PREP) should be saved <4982> (V-PPP-APM).

3rd mention of the word church. first time people are being "added to the church". RWP: "added" imperfect active - kept on adding.
Here apparently the church either has already existed (maybe since Jesus resurrected? not necessarily. we'll see about that in a moment) and people are being added to it? according to Robertson's Word Pictures. "To the church" (τη εκκλησια) is not genuine. Codex Bezae has "in the church."

According to RWP: Those that were being saved (τους σωζομενους). Present passive participle. Probably for repetition like the imperfect προσετιθε. Better translate it "those saved from time to time." It was a continuous revival, day by day. Σωζω like σωτηρια is used for "save" in three senses (beginning, process, conclusion), but here repetition is clearly the point of the present tense.

But note that it was still not called a church in Acts 1:15 for some reason (probably a good one).
Hence I think we have the good reason for the popular idea of the creation of the church on the feast of pentecost in Acts 2.