1. A Word Study Ekklesia Matthew 16:18
Profitable to study NT use of ecclesia
Start you on a word (or topic) study.
Not explain how but give a general starting point

2. Ekklesia - Defined
From a compound of G1537 and a derivative of G2564; a calling out, that is, (concretely) a popular meeting, especially a religious congregation (Jewish synagogue, or Christian community of members on earth or saints in heaven or both): - assembly, church.
Strong's definition
Translated – assembly or church
Part after especially is definition of ekklesia when translated church. Ekklesia is not in and of itself a religious term.

3. Ekklesia - Defined
1) a gathering of citizens called out from their homes into some public place, an assembly a) an assembly of the people convened at the public place of the council for the purpose of deliberating b) the assembly of the Israelites c) any gathering or throng of men assembled by chance, tumultuously
Thayer's definition
This is the definition of the word

4. Ekklesia - Defined
d) in a Christian sense 1) an assembly of Christians gathered for worship in a religious meeting 2) a company of Christian, or of those who, hoping for eternal salvation through Jesus Christ, observe their own religious rites, hold their own religious meetings, and manage their own affairs, according to regulations prescribed for the body for order’s sake 3) those who anywhere, in a city, village, constitute such a company and are united into one body 4) the whole body of Christians scattered throughout the earth 5) the assembly of faithful Christians already dead and received into heaven
Specific for church define – has led to some confusion, but this is how the word is used when translated church. It is not the definition of the word.
Does not mean "God's called out of the world"
NT use will help us see this

5. Assembly - Defined
The act of assembling. The state of being assembled. A group of persons gathered together for a common reason, as for a legislative, religious, educational, or social purpose.
The American Heritage Dictionary

6. Singular Form Jerusalem A Named City Group Addressed Acts 8:1; 11:22
Acts 13:1; Rom 16:1; 1Cor 1:2; 2Cor 1:1; 1Thes 1:1; 2Thes 1:1
Group Addressed
Acts 20:28; Rom 16:23; 1Cor 6:4; 14:5,12,23; 1Tim 5:16; Jam 5:14
Acts 20:28 Take heed therefore unto yourselves, and to all the flock, over the which the Holy Ghost hath made you overseers, to feed the church of God, which he hath purchased with his own blood.

7. Singular Form Individual Group Universal Represented in Local
Acts 11:26; 14:23; 1Cor 14:4; 1Tim 3:5,15
Universal Represented in Local
Acts 5:11; 1Cor 10:32; 11:22; 15:9; Gal 1:13; Phil 3:6
One Universal
Eph 1:22; 3:10,21; 5:23,24,25,27,29,32; Col 1:18,24
In Homes
Rom 16:5; 1Cor 16:19; Col 4:15; Phile 2

8. Plural Form
All Individual in a Named Region Judea – Gal 1:22; 1Thes 2:14 Galatia – 1Cor 16:1; Gal 1:2 Syria & Cilicia – Acts 15:41 Macedonia – 2Cor 8:1 Asia – 1Cor 16:19; Rev 1:4

9. Plural Form All in a General Area
Acts 16:5
Not Definite Region, but not All Individual
Rom 16:4,16; 2Cor 8:23; 11:8
Some of all Individual
1Cor 7:17; 2Cor 8:18,24; 11:28; 12:13; 2Thes 1:4

10. Assembly
Acts 19:32 So then, some were shouting one thing and some another, for the assembly was in confusion and the majority did not know for what reason they had come together. Acts 19:39 But if you want anything beyond this, it shall be settled in the lawful assembly. Acts 19:41 After saying this he dismissed the assembly.