SB 1.2.16: O twice-born sages, by serving those devotees who are completely freed from all vice, great service is done. By such service, one gains affinity for hearing the messages of Vāsudeva.
The servants of God come to propagate God consciousness, and intelligent people should cooperate with them in every respect. By serving the servant of God, one can please God more than by directly serving the Lord. The Lord is more pleased when He sees that His servants are properly respected because such servants risk everything for the service of the Lord and so are very dear to the Lord. The Lord declares in the Bhagavad-gītā (18.69) that no one is dearer to Him than one who risks everything to preach His glory. By serving the servants of the Lord, one gradually gets the quality of such servants, and thus one becomes qualified to hear the glories of God. The eagerness to hear about God is the first qualification of a devotee eligible for entering the kingdom of God.