The Bible tells us that we can know, I John 5:13, "These things have I written unto you that believe on the name of the Son of God; that ye may know (realize) that ye have eternal life, and that ye may believe on (in) the name of the Son of God."
Mark 16:16, "He that believeth and is baptized shall be saved; but he that believeth not shall be damned (condemned)." Christ established His church (ye are a chosen generation, a royal priesthood ... I Peter 2:9-10) to save mankind. Matthew 16:18, "And I say also unto thee, that thou are Peter, and upon this rock I will build my church; and the gates of hell shall not prevail (succeed) against it."
In Acts 2:1-47, that church (the Lord's church), was established. Remember He said, "I will build my church." 'My' is a personal pronoun that shows possession. See Ephesians 1:21, stating "Far above all principality and power, and might, and dominion, and every name that is named, not only in this world, but also in that which is to come..."
The church belongs to the Lord, this is why the Lord adds you to the church and not man. See Acts 2:47 ... and the Lord added to the church daily such as should be saved. (Note: You cannot join the Lord's church, He adds you and your name is written in Heaven, Luke 20:10. Colossians 1:18 says, "And He is the head of the body, the church..."
Romans 16:16 ... The churches of Christ salute you. Ephesians 4:4, There is one body (church), one Spirit, even as ye are called on one hope of your calling; see Ephesians 1:22-23, and hath put all things under his feet, and gave him to be the head over all things to the church, which is his body, the fullness of him that filleth all in all. Ephesians 4:5, One Lord, one faith, one baptism. Verse six, one God and Father of all, who is above all, and through all, and in you all.
Let us always pray that the Gospel will be preached in all simplicity and God's divine word will have free course in the life of mankind.