Prabhupāda:
First of all, the quality. The quality of Christian is that he must obey the ten commandments. If he does not obey, then where is his Christianity? That is stated, guṇa-karma: by quality and work one becomes Christian or Hindu or Muslim. There must be the quality. But when the spirituality develops either from Christianity or Hindu or Muslim—it doesn't matter—then there is. Find out that verse, sa vai puṁsāṁ paro dharmo yato bhaktir adhokṣaje [SB 1.2.6]. Read it. This is the ideal of equality.