like Abraham received the Lord

Genesis 18

v2

And he lift up his eyes and looked, and, lo, three men stood by him: and when he saw [them], he ran to meet them from the tent door, and bowed himself toward the ground,

Abraham received these three who appeared in the form of men, with great hospitality.

Hebrews 13:2

Be not forgetful to entertain strangers: for thereby some have entertained angels unawares.

We should receive the Lord's people and show God's love to them, as if we were serving God Himself, because that is the way the Lord will look at it.

Matthew 25:40

And the King shall answer and say unto them, Verily I say unto you, Inasmuch as ye have done [it] unto one of the least of these my brethren, ye have done [it] unto me.

Especially when there are true believers, we should receive them and help them in the Lord.

Romans 15:7

Wherefore receive ye one another, as Christ also received us to the glory of God.

We should still receive others, whether or not we can be sure that they are doing everything right (Romans 14:1). We should not assume evil things or jump to conclusions (1 Corinthians 13:5,7, James 2:1-9). If the other person is know to have had certain sins in their past, or even if we see things in that person that are wrong, we must not holding that against him or treat him like a second class Christian. We should should forgive him as Christ has forgiven us.

Colossians 3:13

Forbearing one another, and forgiving one another, if any man have a quarrel against any: even as Christ forgave you, so also [do] ye.