Can a church prepare herself as holy?

The answer is No

I) No because — All people have sinned and as the Bible puts it “come short of the glory of God”. It is the Lord who has to take the initiative and call us,

“Who hath saved us, and called us with an holy calling, not according to our works, but according to his own purpose and grace, which was given us in Christ Jesus before the world began,” [2 Timothy 1:9; KJV]

…After that it is upto us to accept or reject that call. This is what we refer to as “Free Will”. This is precisely why the scripture states:

“To all that be in Rome, beloved of God, called to be saints: Grace to you and peace from God our Father, and the Lord Jesus Christ.” [Romans 1:7; KJV]

The concept of man accepting or rejecting the call of God has been emphasized in various other place eg “For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life.” [John 3:16; KJV]

We are made acceptable to God, as the righteousness of Our Lord is credited to your account. This is Imputed Righteousness.

Over time, as we grow in the Faith, Imputed Righteousness has to give way to Outworked Righteousness. For the Bible says — “For as the body without the spirit is dead, so faith without works is dead also.” [James 2:26; KJV]

This means that the Church (body of true believers irrespective of denomination), will have to act out what they believe in. For the Bible states -

“Follow peace with all men, and holiness, without which no man shall see the Lord:” [Hebrews 12:14; KJV]

So,

1. God the Father calls us, based on His foreknowledge   And
2. We have to accept His call