This question arises because of a matter of confusion in understanding several biblical texts. On the one hand, it seems presumptuous (or legalistic) for a local church to ask more of a person than God does for membership in His Church. On the other hand there are multiple texts of Scripture that require local churches to require more of their members than God does for His. Why is this?
To answer, we should probably establish the case that it is the Scriptures that require of local churches that they require more of people than God does. Where does the Bible say anything like this? Before we answer that question, let's consider a few parallel ones. First, would it be right for a local church to accept a person for membership who professed to believe in salvation by grace through faith in Christ but who also refused to give up a lifestyle of illegal drug distribution and murderous enforcement of the turf involved? Would it be right for a local church to accept a person for membership who was professing faith in Christ but refused to stop sleeping with other people's spouses? Would it be right for a local church to accept one for membership who professed faith in Christ but also rejected the responsibility of living under the local church's constitution, by-laws, statement of faith, and leadership authority? Would it be right for a local church to accept one for membership who professed faith in Christ but also professed to believe that Jews and Muslims are saved by their kind of faith? Would it be right to accept one who professes salvation but also rejects the Bible as the Word of God? Would it be right to accept one who professes salvation but refuses to be baptized according to Christ's command?
If your answer to any of these possible scenarios was no, then you are already in agreement with me that the local church is under a mandate to require more of its members than faith in Christ.
Now let's look into some biblical texts that require the local church to do this. First, there is Matthew 18 which demands of the local church that it put out of its fellowship anyone who refuses to repent of any sin that has become a matter of serious confrontation between two believers. This demand boils down to the fact that people who profess faith for membership in a local church must also be ready to genuinely repent of any failure on their part that has created a breach between them and another believer. This is more than God requires in the Gospel of by grace through faith alone. Then there is 1 Corinthians 5. This text required of the Corinthians that they reject one of their own from their fellowship for some form of sexual misconduct. This boils down to a requirement for church membership that adds to faith in Christ the requirement that one not engage in serious sexual misbehavior with impenitence.
The confusing question is why this should be so. Why does God's Word tell the local churches to require more for membership in their assemblies than He requires for membership in His Grand Assembly? The confusion is erased when we understand a basic reality: God requires only genuine faith for salvation and membership in His Church and He Who sees the heart knows when it has been exercised. The local church, on the other hand, has no ability to see the heart. Therefore it has no way of knowing whether faith exists or whether the profession is a deceit. Thus, God has commanded those who cannot see the reality of faith in the heart to require some level of visual reality in the lifestyle. Is this a legalism? Obviously not, since God has commanded it. However, it can be done legalistically. But, whether done in legalism or under grace, it still stands as a divine imperative which the local church can only refuse by rebellious rejection of God's instruction. Therefore, because God is the only one able to see if His requirement for membership in His Church has been met at the heart level, the local church must require more than He does because of its inability to see the heart.
But why should this be so? Why does God not let the local church simply settle for a good-faith acceptance of a person's profession of faith? The answer to that probably lies in the nature of the local church as a visible representation of the invisible Church of God. As a visible representation, it behooves the local church to have standards in place that foster an approximate picture of the invisible Church. The local church cannot, technically, compel genuine faith because it cannot tell if it exists. But, to approximate the reality of the universal Church, the local church can have some behavior requirements in place that allows the world to see an approximate representation of God's plan.
Therefore, as long as a local church preaches and teaches clearly that behavior is not the foundation of salvation, it can insist that behavior springs from the foundation of salvation. Thus it behooves the local church to insist of its members that their lives not bring evil report upon the Word of God and the Gospel of Christ. And it behooves that church to reject any who refuse to depend upon the grace of God to produce lives that are in harmony with the Truth.